1. Alarm clock
2. Life support
3. Colonies
4. ISRU
For 2, 3 and 4, I always really liked Roverdudes mods (MKS, LS, Wolf, ...). For me, it would be awesome if something like that was in the game (gameplay and parts).
5. Asteroids and comets
6. Telescopes
7. Transfer Window Planner
8. Better TWR readings
9. EVA construction and repair
10. Can't really think of a good one, so gonna go with: better...a lot of things (fairings, rovers, SAS, ...)