Name the hardest realistic modpack you can make without RSS/RO/RP-1

The title is pretty self-explanatory. Finished light modded KSP (Vanila++), mastered eveything in the game. Then started RSS/RO/RP-1 and got dissapointed. Dont like RP-1, due to lack of freedom of action and sandbox'ish aspect of the gamplay. Want to play career, but definetely not RP-1. As well as I want to keep IVA gameplay possible. What mods would you suggest for somewhat hardcore and realistic experience if not RSS/RO/RP-1.
I know Kerbalism and Principia would fit well, what else can be added?