This generation broke a lot of conventions. Last gen *sort of* broke the 3-pokemon evolution chains. This one legitimately broke the "legendary pokemon don't evolve" one. So there's room enough for science AND magic in the Pokemon world.
Most of the Z-Moves I'd like to see played with expand on 4th gen concepts. It's not so much a nostalgia thing, I don't think... I don't think... but I do think the unique moves and pokemon from gen 4 are worth revisiting. Spacial Rend, Roar of Time, & Shadow Force, for instance. An exclusive Z-crystal for Deoxys that has a different effect based on form would be cool. It's tough for me to get excited about z-crystals for standard pokemon just because I feel like they're so rarely called for. Although, I do plan on using the Kommo-o one, it's mostly because I raised a competitive Kommo-o and can't for the life of me figure out what it should hold. Golurk already has a choice item. Although I'd like Mimikyu to have one and it likely will. I wouldn't be disappointed to see Aegislash with one, either, but it's unlikely.