I would have to go with the Fallout universe - there's just so much potential there.
Also, Sid Meier's Pirates!, although maybe a little less cartoonish, and with a bigger map with more area to explore.
I can't believe no one has mentioned Firefly yet...although that might be a bit like an MMO version of Privateer, which was also awesome.
I LOVED Pirates!
I laminated the map and used to consult it all the time.
Even in CGA, the graphics were great.
I played Pirates of the Burning Sea, and it actually has quite a bit of the Pirates! feel to it.
You might give it a go since it is also FTP.
Firefly would be a definite!
Otherwise, I think World of Starcraft is loooong overdue.
Oddworld would make an interesting place to hang out. Loved Stranger's Wrath (played it on Xbox and then I purchased the DL on the PS3! )
Any of you "mature" gamers remember one of the first EA titles called "Starflight"?
Now THAT would make an awesome MMO.
Really it had all the makings of one, even back then.
And the sequel was pretty good too!
Gotta love the Spemin...
Some of my best RPG stories come from that game. From that game, I learned to always hit the deck whenever the lights go out. *LOL*
What I like about it, is that very few modifications would need to be performed to make it work as an MMO. You already have replacement clones ready to step in when your character dies, and there as lots of room for intercharacter interaction and play not seen in other MMOs.