The most interesting part of Z-moves were the alternate effects that status moves were given. At least they took up an item slot, which is more than you can say about any later gimmick.
Z-Moves suffer from an implementation problem on top of everything else. Remember Light That Burns the Sky? The move that sucks all the light from the world and throws it in a Death Ball and causes the trainer to stagger backwards? That must be the most powerful move in the game, right? Nope! It's actually less powerful than picking up a Pikachu and throwing it at someone:
I mean, this formula could honestly work for Pokemon imo. At least when it comes to the exploration and adventure element, as it could bring back a sense of wonder to the series.
Pokemon has always benefited from the "and then play it again with a different team" advantage, adding alternate endings seems like a logical progression.
They were just a way to sell you mewtwo and charizard again. Everyone of them could have been their own independent thing with balanced stats but weren't. Paradox forms are just megas yet again. And I would still rather have had the time and energy spent improving neglected lines. Not just temp alt forms we'll never seen again over and over.
