A user friendly save system, where there's not like 3 or 4 different kind of saves. Which don't really make a whole lot of sense to the lay user.
Planet Shine by default
Decent stats to actually understand whats going on, ie. kerbal engineer.
Automated systems (as progress in career), kind of like mechjeb. But taken further, should be able to have boosters land spacex style if progress far enough. Or have miner probes, ferry resources automatically even if not focuses. Espeically as resources to build massive ships more important now. Turning KSP into a kind of KSP Transport Tycoon, or Sid meier's Sim game in later stages. Build and prove something works, sort out basic logic. System will repeat in background.
Procedural fairing, like the procedural fairing mod - but more user friendly and less janky.
Different views in normal gameplay, airflow for example. Something that visualises whats actually causing the issue, Cities Skylines great example for in game data visualisations.
Just basically take all the best QOL mods and merge them in and take them a bit further.
Sneaky edit as remembered stuff...
Laser highways, set up a network of laser outputs across the solar system and beyond to push space craft around the systems. This is the most realistic (IMO) economic end goal for commercial stellar and interstellar transport.
