I loved Elite on the C64 as well as the first couple of Wing Commander games (and I'd wanted to give Privateer a go but my PC at the time just wouldn't run it.) I think some space-flight sims would be cool, something like Privateer where you could trade, be a pirate, or what have you. There could be an over-arching storyline, but still sandbox it if one wants to play it that way.
The first Mech Warrior game (I never played the later titles) had something like this, with the main story always waiting for you to pick it back up where you left off. And until you did you could just jump across the galaxy earning money and buying up bigger and badder mechs.
And I'm sure I've said it before but I'd LOVE to see a sequel to Crimson Skies. I loved the world it was set in and definitely have been wanting more of it (hmmm...maybe I'll fire it up on the 360 and give it a go-again.)
I plan on picking up Dark Void at some point, too. Seeing it at last year's PAX had me really wanting to be the Rocketeer beating up alien-baddies. I wish the 8-bit version was coming out on an actual cartridge instead of just the DSi-download (since I have a DSLite.) It looks like a fun visit to some classic gameplay.