Whenever flying in sector space, or in a planetary system, or on the ground for that matter I always felt boxed in and constrained. It was nothing like X3 where space seemed huge. I didn't think think it was possible for an MMO to have such a scale until I encountered Infinity from Inovae studios. They just released a tech demo of their procedurally generated galaxy, where each dot on the map is an explorable star, and each planet can be landed on. You can see part one here
This engine was pretty much programmed by 1-8 devs, and its being designed for a MMO as well. If there's any way to license the engine or incorporate some of its design principals into your own, I'd ask you to consider it. At the very least exploration sectors should be handled this way. The seamless transition from space to ground was shockingly good.
Anyway I know its a longshot but I wanted to get this out there that space and ground can be seamlessly joined without too much undue instancing, and yes I do realize in the tech demo there's only one person in the game .
Page with the videos if the above links don't work
Th trouble with infinity is that it looks the part and pushes all the buttons but it is still very much a demo.
Egosoft's 'X' games on the other hand are the best space game /simulations ever made. Head and shoulders above anything else. Too many times I have heard rumours that they will turn it into an MMO only to have my hopes dashed. I would be on that like stink on a turd if it ever came to pass.