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Mega Evo: Give a non legendary higher base state totals that most legendaries at the cost of a held item.
Z-Move: Free kill
Dmax: Only increase base HP, add 50 (on average) base power to a move (capping at 150) Make it's accuracy 100 and make it's effect change the weather/field. Only last 3 turns.
Dmax is mechanically superior to any other gimmick, it adds dept and strategy to battles and denying that just makes you look stupid...
That being said, fuck gimmicks all gimmicks are unnecessary.
Please god just let gen 9's gimmick be open world.