Think about the main gripes of X3 -
Autopillock
Static political domains
OOS combat
Sector/gate structure
Fleet equipping/management
So, dump the characters, maybe keep the universe, maybe change the way of getting round. I think someone else suggested the gates failed so you need to find the sectors yourself (which means you'll need a properly last ship!!!) and deploy new jump markers. I like that idea.
And OOS will change but maybe not that much (yeah, it's limited, but the reasons for it have not completely disappeared). I think the races may well war with each other properly - that'll be fun
Autopillock will have upgraded gerbils (maybe even as far as some lower primate) but still not as far as a higher form of primate
And, finally, a proper, working, easy fleet control interface! Yippee!!
But they're all fairly easy predictions - let's face it, Egosoft will have been looking at the limitations of the current engine (and it's strengths) when looking for a functional design for the new one.
It'll be a space game. It'll be free form (pirate, smuggle, trade, war monger, bounty hunter, whatever). It'll have a real economy (ie everything needs raw materials to be produced and these raw materials will need to be shipped round either by NPCs or player - energy will will probably be free to NPC and require some form of resource for player, as it does now).
It'll be brilliant
Tim