I would LOVE to see Air Traffic Control get some serious care and affection.
There's loads of AI tools lile BeyondAtc and Say Intentions on the way, and of course plugins like VATSIM, but I would just settle for a built-in ATC that doesn't want to fly me into a cliff, or make me wait on a taxiway for a bus that has inexplicably decided to park in front of a 747.
I'd also really like some community multiplayer tools. The old games had proper servers you could join - doing fun activities like landing challenges and flybys. I miss those spontaneous connections with other pilots.
But aside from all of that, I just want something that makes me feel like I can do that thing which I'll probably never get to do... Make a hunk of metal that weighs 35 double-decker buses go up.