I'm back from camping and I just moved today to the beautiful Columbia River Gorge area in Stevenson, WA. Looking forward to getting settled.
But back to STABmons...
Similar note, grant Psychic STABs to Darmanitan - again since it can change form mid-battle, and it gains nothing notable. Also the possibility of granting Flying STABs to Shaymin-L - while it can't switch form in battle, it also doesn't lose it's Flying moves when it changes form like Rotom does.
Really, Rotom (and Arceus, if people ever play this in Ubers) is the only form problem due to it being simply overpowering with access to so many moves.
I'm liking the Shaymin-gets-Flying-moves idea since that makes sense to me.
I don't think I'll do Darmanitan with Psychic, Meloetta with Fighting, and Castform with Ice, Fire, and Water because those formes only appear
during battle whereas Shaymin can keep Flying-moves it learns while it's Shaymin-Sky.
-Basically, if it only changes forme during battle, it doesn't count towards learning that new forme's STAB moves because that type doesn't stay with it after battle. (Darmanitan [forme is a result of ability that kicks in during battle], Meloetta [forme is a result of using Relic Song], Castform [forme is a result of in-battle weather])
-If it changes forme outside of battle and can carry over that STAB, then the other forme gets access. (Shaymin-Sky -> Shaymin, similar to pre-evo carrying over STAB attacks to new evolutions)
-If it changes forme but loses its STAB attacks when it changes back, then it doesn't get access to other formes' moves. (Rotom-W can't learn Ice moves from Rotom-F because regular Rotom forgets Blizzard when Rotom-F changes back)
I hope that makes some sense... I just don't want it to look like I'm making arbitrary rules. Feel free to bring up different ideas, however.
P.S. Arceus is different, but I don't even want to touch Ubers right now.