- Which Poké could fill Roselia's niches as a bulky Grass type, and as a Spikes stacker?
It is basically summarized as thus. If you want sleep and bulk, pick Tangela. If you want Spikes and bulk, pick Quilladin. If you want Specially Defensive Bulk, pick Gogoat. Roselia provided all three along with status absorption thanks to Natural Cure. Gourgeist-XL is also a viable replacement, but he didn't fit into my nice explanation.
For the sake of perhaps a new conversation (don't kill me for this, however, bad it may be) I kind of want to mention both Ivysaur and Gloom. I remember somebody mentioning them before on the forum and they were unceremoniously dumped because Roselia was just better. But if Roselia were to leave they might have a chance to shine. They have the same Special Bulk (actually more due to 10 more HP) and same typing, meaning a resist to Fighting-Type moves, they also both have Sleep Powder, and better physical bulk than Roselia. Just lacking in power, spikes, and a resist to scald burns and Ice Beam freezes. Both would have to sacrifice some Defense Evs to get a OHKO on a few mons with Energy Ball or Giga Drain.
There may also be a niche for Meganium now as with Aromatherapy and equal 100 in both defensive stats, if you need a bulky grass-type that doesn't mind Toxic as much as the ones previously mentioned you may want to give Meganium a glance, although it misses out on Sleep Powder.
Spike-Stacking has already been stated fairly easily. Nothing new to add unless you want me to talk about Tentacool and Koffing?
- How it would fare its replacement in the meta?
Quilladin will probably benefit the most seeing as it becomes the bulkiest Spike stacker in the meta, Gogoat will rise as it becomes the most Specially Defensive Grass-Type, and Tangela would benefit the least as its a physical wall like it always was. It just no longer has the alternative.Ivysaur and Gloom both may become D-ranked, big whoop. And Meganium might see some use.
- Which pokes would have increased or decreased viability thanks to Roselia not being on PU?, Which could be their niches now?
The replacements as stated above for the reasons above. All Specially offensive Water-Types would love to be able to either Surf or Ice Beam and get a KO on things. Physical ones might lose some ground as Tangela becomes more and more popular meaning they don't fair quite as well with Ice Fang. Offensive Grass-Types as no more x4 resists unless Gloom and Ivysaur catch on more than they should. As well as Electric-types like Raichu and Zebstrika as they no longer have one more bulky Grass-Type to worry about.
I suppose some Defoggers may lose their momentum as they no longer have Spikes to be immune to so they lose one small reason to use them. Also since Stealth Rocks become the main Entry Hazard to remove Vibrava and Wartortle as the only two removers that aren't weak to them might see a bit of an increase, but this is just me trying to offer something new to talk about now.
- Roselia moving could motivate some additional rises from FU or drops from NU?
Roselia is basically like a Spiking Vileplume so no change from NU. FU might lose some Water-Types (like Basculiin and maybe Golduck as it was a Specially offensive Water-Type) it held, but PU has plenty of those (Simipour). Quilladin and even Whirlipede might rise as PU would be desperately in need of Spikes, but I can't see either really gaining too much steam as they both missing something (Sleep Powder for Quilladin and longevity for Whirlipede). I can't imagine too many others leaving.
- Which Pokés now will run another moves since they doesn't need to deal with Roselia?
Roselia wasn't particularly bulky on the defensive side so most mons could just focus on that rather than SE coverage.
- Which kind of playstyles could be favored by Roselia leaving PU?
Stall would take a hit as it loses a Specially Defensive mon along with a reliable source of poison in Toxic Spikes. Hazard Stacking as everybody else said would also be heavily injured. Hyper Offense would probably see a rise as people who loved Spikes would basically be stuck picking up a suicide spiker if they wanted to keep it.
- Is a Roselia Ban probable after the possible ban of Carracosta and Barbaracle?
Roselia is practically the epitome of S-Rank. It can do what it does without any extra support. Its not broken thanks to a rather low physical defense allows many mons to take advantage of that and outright kill it. Its good, even great for tier, but far from broken. Its a hazard stacker, which is weak to the majority of the hazard removers. Pelipper, Swanna, Torkoal, and Avalugg all carry an SE move and Armaldo just had such huge base ATK stat and Knock-Off that Roselia was scared of him. Its not impossible to stop it from doing its job.