Hey guys i'm kinda new here, but i have spent a fair amount of time in game. I find that most of what i do is, go to planet, get research rinse and repeat (If i'm not just going for a challenge). Now the fun part is that each planet is different, so figuring out how to get there, and what you want to get there with etc, is the fun part. But you get there essentially and you just do research and you send it back. Hopefully its a return mission so you at least get the joys of reentry. What i'm suggesting is resources, lots of different resources scattered over different planets with different rarities and the ability to use those resources in many different ways.
To help you understand lets assume you can make supply runs to planets by collecting ore and sending it home. I make it sound simple but you assume you have to send the parts their to find the ore, then land the parts to mine the ore, after store it for transport. Finally safely get it back home. You make some money and a little science along the way. Also you should be able to fabricate land bases on other planets by means of fabrication. Use your resources to build a little base. Launch small rockets from the planet you are on, build rovers, do research on strange rocks you found etc. I just think this is a foundation upon which a lot of things can be built upon.
A color palette (give us red, blue, green, etc for ships.)
Make it so that the land facilities can calculate interception nodes. Maybe a slow kerba computer that makes calculations that aren't always the most accurate but can set up maneuvers from home just like in real life.
An in game news paper, while in career mode it would be nice to read about your company in the paper and some other comical stuff that is happening in fictional kerbal society.
A black hole. (O_o) Ok this one is big, but how fun would it be to see how close you can fly to a black hole......