Came up on IRC, now I'm thinking about it and brainstorming.
Not that I feel like even touching the idea of implementation with a 10-foot pole.

-
Fix ingame joining. This is important. Probably the most important thing, as you can do a lot of the other stuff with SEXPs.
- Longer update times. Necessary if you want the first "M" in MMORPG. Otherwise you're stuck with the usual limit of players.
- Add the ability to save data on players to the code to some sort of file via SEXPs.
- Implement an alternative chat system.
- Probably give some more low level SEXPs, like a "create-ship" SEXP.
- Add the ability to have some sort of means of monetary exchange (Not score; score should be equivelant to powerlevel points)
- Which of course means you'd have to SEXP in the variable with the above player-data save SEXP stuffs.
- Implement code to load the player's curret money level and offer them ship choices. Possibly allow use of SEXPs in the ship selection screen/briefing?
- Set things up so that only nearby ships are displayed/positions updated
That gives you a 'mission' where players can enter, leave, talk with NPCs, gain items...well, that's pretty much it. OTOH, what more is there in a MMORPG?

Limitations:
- One map. Duh.
- If you want to change something, you have to take down the server.
- The above assumes that a lot of stuff is hardcoded; you'd need an items.tbl for any sort of flexibility.
- Although, interestingly enough, since it'd be necessary to build the player-item-save code it could probably be made to allow new items to be added dynamically (ie without restarting the server). How you'd work that in with SEXPs, I'm not sure.
- I have no idea how much of the code would have to be made to be more efficient and recycle better. There are some variables that are just constantly incrememented. Not good when we're talking hours and hours of uptime.
- No real way to know what items you've got except with the advanced chat system, unless some sort of windowing stuffs is added to the game (either through another state, or by grafting the new GUI in-game - which is entirely possible but raises mouse control issues.)