Hopefully we get a system for automatically assigning numbers and serials to aircraft, Heck, I'd consider that almost mandatory for a new game, and I'm surprised that some other sims didn't have that feature until quite recently..
In terms of more "wishlisty" things, I quite enjoyed IL-2 1946's style of applying nose art, would love something like that in a modern sim! And of course, custom cockpit photos are lots of fun and I enjoy it in all the games that have them.