Fusion or armour could be nice, I've certainly discussed ideas for both here.
I find armour a very interesting gimmick concept - I'd talked about an idea for such a mechanic earlier this year here:
https://www.pokecommunity.com/threads/425769.
Aesthetically, armour could be executed in a variety of ways: you could go with literally metal, Mewtwo-style armour, or (these two being ideas from my thread) have it be made of small pieces of crystal that grow over the Pokémon's body like blue vitriol into cool crystal armour,
or it could be organic armour-like growths sprouting from the Pokémon's body (made by, say, some symbiotic armour-producing Pokémon amoebas)...
Mechanically, my idea for armour was boosting the subject's low stats, but you could imagine it as any number of other things. The visual and conceptual aspects of gimmicks are fun, but so is their mechanical aspect. For example, Dynamax = double HP, strong moves with such and such effects, lasts 3 turns. Very elegant and fun to use.
Bottom line, thinking in mechanical terms easily leads to lots of cool gimmick ideas. For example, a gimmick that lets you boost a Pokémon's selected stat, or a gimmick of adding an extra type of your choice to a Pokémon. Or a temporary gimmick type you can turn a Pokémon into, like a type that resists all other types or a type supereffective against all other types...