Fluidmorphs as a whole. The non loyal ones can go into ANY water deck and just cast everything with a crown or fork. If Najarin FF goes then they should too. Or at least be nerfed heavily like making them all loyal and unable to use a fork or crown. Corus and golden heptagon are fine though and might actually get some use if that were the case.
Zamool and Aerdak/droniq if not fluidmorphs outright.
Stuff like ursis,chaor tf,maxxor Pop,headmaster and illexia are powerful and significantly better then others but arent nearly as gamebreaking as the rest.
The only other things I could see a ban on are Imaginary walls and maybe MAYBE melody of the meek. Although meek is counterable in different forms its not fun to play against.
If were breeaking the meta then SCR,Deadwater,and Bodals dagger.
Bodal's being on there because 10 extra damage every attack with no downside. (Look at mipedian fulergate) sure the downside being you cant use stat or element attacks but with stuff like retalitory strike,reverberate and primal, who cares? Element and stats can be stopped ,base damage is base damage.. (azaia and numin just stop decks outright)
It's powerful and its used in most top decks for these reasons.
I dont even think there should be a ban list but if you really want one. Thatd be it and why.
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"What are you gonna do? STAB ME!?" -stab Victim