If I had the time to get a few colleagues in on helping me make a mod. It would be to get a naval-like orbital gameplay with large battleships etc. Not free space fighting, just large scale orbital. Gas giants would be a great canvas for working that out. The thought of dealing with selecting those units with ground units below the way it is at the moment grinds my motivation to a screeching horrible stop, followed by a derailment and some explosions.
For myself I'd like to make Orbital gameplay a lot more realistic (with actual orbits) and units that stay as close to satellites as possible, rather than spaceships. I'd be interested to see what you make however, just to see what a different direction for PA does for the feel of the game.