I am a fan of Career mode from KSP but am slowly coming around to the idea of a single mode to unite them all...
however I think this should be accomished with sliders & boxes through the games difficulty interface.
I like early Game difficulty centered around currency limitations & the efficiency of design through graduated part limitations.
Most of all I am a fan of Contracts. The procedurally generated kind that continue to add spice to the system.
I personally feel that Contracts as a supplement to Missions would be off the planet. Space X & NASA will take contract work, KSC should as well.
There seems to be little structure with regard where to solicit these suggestions. Many are repeated enough that I'm sure they gain traction, others not sure.