Maybe a Pokémon that evolves when you catch more of the same species? Diglett and Magnemite should've evolved via this method imo, and the remaining two (since both are made up of three Diglett/Magnemite) should have disappeared to combine into one. And maybe leveling up after surfing on it often?? The Pokémon can evolve from something that's not the best swimmer to one that has honed its skills in the water. Would be interesting!
They dislike conflict, and if they need to escape from enemies, they will roll away.
I don't want to see anything gimmicky ala flip your 3DS upside-down for Malamar, but Gen VI was going the right way with things like Eevee to Sylveon (knowing a certain type of move, not the amie stuff), and Pancham to Pangoro (have a Dark type in your party) would be cool to see. They're not gimmicky and wouldn't be too hard to achieve, and actually make some sort of sense lore wise.
Evolution based on physical attributes like individual weight/height. In some games there was this guy were you could bring Pokemon you caught (usually magikarp) and he commented on their lenght/size and gave you a reward if they were big enough, so there was obviously some hidden stat that assigned a length/size to every Pokemon of that species you caught.
Make it related to that Pokemon lore, for example they live in packs and only the biggest/strongest gets to evolve and becomes pack leader.
To avoid it being frustrating, if they can evolve then they have a certain number of IV guaranteed, which ties back to the lore: if they're weak they would have had low IV anyway.
More evolutions related to lore in general: like mantyke should have become mantine with a remoraid in the party.
Evolution based on what you feed them to influence their stat, a bit like Tyrogue but maybe with some minigame to make pokeblocks/pokepuffs/potatoes/whatever.
Ngl, I like gimmicky evolutions. But I agree with Inkay's evolution being the worst of those "gimmicky" variety.
Honestly, I'd like to see an evolution that requires to be in a Sandstorm/in a desert-like area to evolve the Pokemon. We've never seen a Pokemon that evolves this way. Maybe it'd be a good evolution for a Galarian Helioptile, or a new ground type. There are plenty of animals in the real world who adapt to harsher environments, and desert environments/sandstorms are QUITE a harsh area.
We've had Pokemon evolve in an electromagnetic field (Magneton to Magnezone, and Nosepass to Probopass), and Pokemon that evolve in a colder climate (Crabrawler to Crabominable), but never a sandstorm.
I also want an evolution that plays around with certain status problems, like a Pokemon evolving by leveling up while being poisoned? I could see this evolution type working for a first stage Grass Pokemon that becomes part Poison through this evolution method. I don't know, I think it sounds cool.