Can't think of much that mods don't already do. But with that I'd probably like some mod-like features added into the game, such as they don't break so often or maybe can be done in a more optimized way.
Visual enhancements, now my pc isn't too powerful, but I find that playing without clouds for Kerbin is almost unplayable. I'm not asking about a full on graphics card melter but some simple cloud layer, which could maybe be improved in the settings for those with better machines.
Sound enhancement. One of my favorite mods is Real Plume, not so much for the improved plume of engines (which is awesome on its own) but mostly because it makes the engines sound like real, powerful rocket engines.
Some kind of life support, even if simple. Snacks is the simplest LS mod that comes to mind, probably something along those lines would be the best as to not over complicate the game.
This one is highly unlikely, but would make me buy it right away. A RO like change of the game. Make it realistic as much as possible.
This one is the one that I don't know if it exists as a mod. Closest I know is procedural parts. The possibility to create our own parts, within the game, not having to resort to extra programs and configs, but an fairly easy and approachable way for everyone to make parts as they want them to be and not be so limited.