This is a down-the-road-type thought, because clearly other things are more important, but, as a bike nerd, I would enjoy more than is reasonable the ability to change more than just wheels and frames. I realize that licensing is a thing and that bike stuff gets obscure very quickly. I wouldn’t expect to be able to perfectly recreate my mid-'80s Gianni Motta with its late-'90s Campy or whatever, but it would be cool to be able to tinker with stem height/length, bar bend, tape/saddle color, saddle shape, tire color (I want skinwalls!), and maybe a choice between black and silver groupsets (dream scenario: c-record option).
Beyond pleasing nerds like me, I think this might open up some possibilities for the social side of Zwift–like era- or manufacturer-specific rides or races, or even just the possibility of making an in-game connection/friend/whatever by riding up on someone with a bike you appreciate. It happens on the real, physical road, outside, so why not in game?
(Alternatively, something to not take seriously: I have a bit of a knack for 3d modeling, so if I could just figure out where the game keeps the models on m’computer and how to set up and export my own models, I would be exactly as happy as clams are, even though nobody else would be able to see my cool bike.)