Banning Tera Blast
Banning Tera Blast would be seen as the least invasive, but most straightforward and precedented, way to restrict Terastallization. To put it bluntly, this does not address the core of the problem or the most focused on aspects of the mechanic, so it seems like a solution that should only be elected if this concept is viewed to be only slightly problematic rather than wholly.
Only so many abusers actually utilize Tera Blast as many simply prefer the additional STAB for other coverage or for making it harder to approach your Pokemon with would-be super effective attacks. Tera Blast is just one subset of the larger topic of Terastallization and this would be the most minimal restriction of the bunch, reserved for cases where only minimal reform is needed if discussion trends in that direction.
limiting the amount of Pokemon on any given team that have access to possibly Terastallize during a battle
One can argue that the layers of unpredictability will be minimized if less Pokemon can possibly utilized the mechanic, potentially limiting it to a smaller pool of Pokemon on each team or even just a singular Pokemon on each team. This can be akin to a handshake agreement made between players on the cartridge, too, that we would essentially enforce as a clause into our mechanic and implement through the teambuilder rather than as a modifier of the battle mode itself.
If the the pokemon who can terastaliize are limited to one, I can see terablast becoming more of an issue down the road, because now the opportunity cost of running Terablast currently will largely be mitigated. The current opportunity cost is that you don't know if you will end up Teraing the mon(s) that you are running Tera Blast on, making it potentially a dead move slot. But with being forced to only run 1 tera mon, terablast can be effectively used with no downside. Though at that point, terablast may still be among the smaller fish to fry. As someone else mentioned, it's worth coming back to this down the road when DLC comes out because more mons may also consider running TeraBlast then, but not sure how relevant this will be then.
Limiting Tera typing to previously existing STAB types
Much like the above option, this would inherently restrict the abuse of Terastallization, but it would not be a full-stop to the possibility for trouble to arise. It would be more possible to keep Terastallization in the metagame without having to worry about Pokemon adopting entirely new typings to shift their match-up coverage throughout the metagame. However, adopting a Tera type within your STABs for a Pokemon with two types can still drastically shift a Pokemon's profile as you can shed weaknesses if timed properly. In addition, granting every Pokemon boosted STAB to the point that it is like they have Adaptability can be seen as problematic in it if itself.
From a competitive point of view, this would be less problematic to the metagame than leaving Terastallization entirely untouched and skirts away from the aspect many believe is the least competitive one. However, it also leaves a lot there that can prove troublesome for the playerbase, especially when this is considered to be present on top of one of the least forgiving power creeps we have ever seen. Implementing this could be seen as something akin to a handshake agreement between players, which could be more plausible as well.
I think the Adapatibility boost is probably still too strong. Furthermore, limiting tera this way hurts defensive cores more. The offensive boosting mons can still change their typing for some defensive merit ( Dragonite can now make itself x2 weak to ice instead of x4, which can be relevant) but now defensive pokemon, while they can still make themselves a mono-typing for improved defense in some cases, this really limits their options I think for checking a runaway Tera Sweeper in an emergency.
Furthermore, as other people have stated, it guts the identity of the mechanic in a less than ideal way, the cool part is being able to change your typing to something completely new. Being able to change to a mono-typing from a dual typing still preserves this idea of typing change to some extent, but it kinda awkwardly falls short, and would leave mono-typing mons with only an adaptability boost to look forward to, which is likely still problematic in and of itself to the point further action is still likely needed anyway.
In the event that Terastallization is seen as a concept conducive to competitive play, bringing out an acceptable metagame state with it involved on a consistent basis, then we would opt to not change the application of the core mechanic whatsoever. This would neglect to address any of the above points about the burdens of type changing on counterplay or the potential issues with the additional strength provided to Pokemon through boosted STAB or a novel STAB type as well. However, neglecting these matters could be seen as acceptable if they do not seem overly problematic to the bulk of the playerbase. This will almost surely be an option in any suspect or vote on the matter of Terastallization because of this and the fact that it is the status quo. It is important to us that many players see Terastallization as a draw to participating in our metagames; while it is the first and foremost priority to maintain competitive integirty and balance, it is also a factor to have generations motivate players to participate and have an identity. This premise is a large driver behind the potential for no tiering action or limited restrictions, and additionally add a potentially higher burden of proof to the outright ban side as well.
Would be ideal, but I don't think it will be realistically possible unfortunately to leave the mechanic as is, and don't think DLC will make the issue any better.
Showing Tera type at Team Preview
This would be the biggest restrictor of the core mechanic and perhaps is the one that has garnered the most discussion. Cons include the fact that it is still not a perfect solution to the guesswork required to play around Terastallization and it would require potential display modifications that are seen as undesirable in many circles. The pros, however, would be that it limits the pool of possibilities for players to abuse the mechanic while setting an expectation what's to come.
From a competitive point of view, it is helpful to know what every Pokemon's Tera type is, but it is still inherently challenging to line-up your counterplay with the timing of the opponent and their use of the mechanic. When a Pokemon's defensive profile can do a flip-flop on command, changing the entire type chart on whim, it makes counterplay as a whole unreliable. Surefire revenge killing methods can be dwarfed by sudden resistances and immunizations while an additional STAB typing or boost to previously existing STAB can tear through counterplay even with prior knowledge of the possibility just because all game will be played in fear of the prospect and it adds a premium offensive bonus that is not seen through other means regardless of what information is disclosed.
This restriction may cut closest to the core of the problem, but it still does not assure the balancing of Terastallization and implementation is controversial.
Helpful, but still too much of a half-measure. Still wouldn't be able to time the Tera accordingly in many scenarios, and in addition it still leaves room for mind games as people can use Dnite with Fire Tera to bluff a certain coverage and set for example only to run another coverage, and terrastallize something else, taking advantage of the information presented to the opponent at team preview.
More work would still likely need to be done if this was implemented, but could be a helpful supplement in nerfing Tera down.
Outright ban
An outright ban is Terstallization will remove the entire concept from the metagame with no strings attached. This can be seen as the "nuclear option" as it removes the entire core mechanic from the current generation of OU, but in extreme cases a nuclear option may be necessary to reach the ideal result. In this case, the ideal result is a competitive metagame and we would resort to this option if it is determined that there is no place for Terastallization in a competitive environment.
There has been a significant amount of buzz about removing Terastallization from the metagame altogether as the alternatives, which will be discussed below in the "restriction" and "no tiering action" sections, all leave things to be desired. They all come with their own respective cons, but the collective pro of preserving some semblance of the generation's core mechanic in the metagame. As for an outright ban, it is the opposite -- if we were to go to this option, we would face the con of it being removed altogether, but have the pro of avoiding a potentially ideologically flawed or inconsistent method chosen as our solution.
In terms of practical outlook, a lot can be said as to having the ability to change defensive typing making finding consistent counterplay to Pokemon an impossibility. Terrstalization fundamentally alters how we approach handling the wide array of threats our metagame presents us, occasionally forcing the metagame to resort to extremes with a surplus of revenge killers to minimize prospects or a surplus of extreme walls to outright blank Terastallization options on more dynamic offensive presences. It is possible to argue that this concept as a whole does not belong in a competitive metagame due to how much it warps how we play and how even with the closest attention to detail, it can be seen as an unreasonable ask to handle both Pokemon in their original state and these Pokemon with altered types. However, it is also true that more experienced players have began to expect specific Terstallizations from specific opposing Pokemon. This at least adds a layer of strategy and a component of prediction to the matter. It is true that there can be some guesswork when it comes to timing and specifics, but it also takes proper usage to reep the rewards of Terastallization.
The move Tera Blast also warrants discussion. It has been serving as a practical tool to a number of Pokemon in the metagame with a STAB boost and being able to adapt to your stronger offensive side. The limiting factor of it is that it does take up a valuable moveslot and it also is only 80 base power. This makes the viable pool of Tera Blast users slightly limited, but there are still a good amount of users overall. We can say that Tera Blast alone as a concept is not the most overwhelming thing we have seen and may be tolerable in a vacuum as of now, but there are still lots of new applications of it popping up that give Tera Blast potential to get scarier by the day in the metagame.
Overall, the main selling point for an outright ban would be that the sudden type shifting is seen as an uncompetitive element of the metagame, thus making the premise of the core mechanic banworthy. It is currently yet to be seen as to if the playerbase views this dynamic as competitive or uncompetitive, but we will use this thread to assess that matter and guide us in proceeding.
Probably will be the best option tbh, and more consistent with Smogon tiering philosophy as of recent (i.e. Dynamax ban). Banning a generational mechanic is of course very controversial and will leave many unhappy, but Smogon still put its foot down for the sake of competiveness last gen despite the outrage. Community growth and interest is an understandable concern, but think about how much bigger Smogon could theoretically be if we also didn't ban everyone's favorite Pokemon or items from Overused. There will always be people shunned by this (I know a lot of people who were upset when Megas were being banned left and right at the beginning of XY) and I think Smogon ought to want to attract like-minded people looking for tiers with the best competitive balance possible, and "Complex" rulesets should usually be avoided as per Smogon's tiering policy and tradition, and for good reason as complex rulesets can be seen as arbitrary if not causing a reduction in accessibility And the thing about all of this is, while it is and can be frustrating at times to not be able to play with a mechanic or pokemon in a certain tier, especially if one really thinks it is balanced in said tier, the reality is Smogon still has at least one avenue where these things can be used (Ubers, Anything Goes, or if people really want and resources are available, can make an OM with Tera available in a clone of the OU (or other ) metagames). There's also just challenging people to a private battle with your own rulesets established, worst case scenario.
I think Tera is a cool design in principle, just very poorly balanced, and I think the best way to balance it would probably need a modification of cartridge mechanics to some extent.