First, I suggest the same idea with number 1 with one difference: The lower stat via Atk/Spa should be boosted more than the higher one, and the lower stat via Def/SpD should be lowered less, only calculated from Base Stats (IV or EV is not considered, same as another Mega Stones).I have a couple ideas regarding Unecro transformations that might work.
1. (cheap method) Higher Atk stat leads to the transformation that gives more Atk and same logic for SpA.
2. Pick one transformation to be shiny and one to be regular, however this might give hints in team preview. You might want to slap random shinies to throw off opponents.
3. 30 Atk IVs leads to the transformation that gives more SpA and same logic the other way around.
4. (too difficult probably) Make two separate items just for this.
I think this is a neat item and advocate it for it to be introduced in some way.
As you know, Necrozma-Dusk-Mane has 157 Atk and 113 SpA, and Necrozma-Dawn-Wings has vice versa (113 Atk and 157 SpA) - but their Ultra Bursted forms have 167 both. The same logic can be applied to Def/SpD (127/109 or 109/127 → 97/97).
So, its stat changes on my first idea would be:
Type: +Dragon (Secondary)
Abiility: Neuroforce
Stat boost/drop:
+10 or +54 on Atk/SpA (the higher one gets the former)
-30 or -12 on Def/SpD (the higher one gets the former)
+52 Spe
(Total: +74)
But the problem is, on this logic, we should additionally decide what should occur when a pokemon with the exact same Base Stats of Atk and SpA, or with Def and SpD. Of course, we can make another rule like "both get +32 or -21 if tied" from average, or "both get +10 or -30 if tied" to keep it a little more balanced, but I'm worried that these solutions might break Metagame Workshop's philosophy, if those exception rules would be added only for Ultranecrozium Z: "Keep your Metagame premise succinct."
To avoid making exception on same Base Stats, I can suggest the second idea: "+32 Atk/SpA from average, -21 Def/SpD from average, +52 Spe (each average is rounded down)". It is reasonable but might be overpowered, and I also doubt this could be coded properly.
But if possible, whether the first or the second idea, I also hope Ultranecrozium Z becomes one of "Mega Stones" (but I think Z-Move feature should be removed from it) , even if it gets banned right after it. Once if the algorithm is coded, the users can unban them on custom games, for suspect tests, for friendly fun game, or for whatever.
(Honestly, I don't expect it is balanced with +52 Spe, no matter how Atk/SpA or Def/SpD stat boosts go on)