Morrowind - I mean a perfectly created world like this could make such a magnificent MMO. I truly think a spin off Elder Scrolls MMO would be excellent.

Mount&Blade - Most of you won't have played this but with multiplayer the battles would just be MIND BLOWING.

How would Mario platformers be online? Mini games?

sunset riders

also soriku, i believe that there was an online zelda lttp on the snes sattelaview

Bioshock

Thats the only game that needs online on the xbox 360.

Virtue fighter 5 on the ps3 as well, ps3 owners need it as well.

Super paper mario online co-op would rock.

I don't really understand why the Wii vesion of Worms doesn't have online, other than not wanting to release it during summer.

AND Metroid Prime games...and basically every game that ISN'T a platformer or any other non-online game.....party games would be fun too with online

Uncharted: Drakes Fortune

All games that don't have online, should have online.
All games that don't have offline, should have offline.

-GT4
-VF5 (PS3)
-Uncharted (maybe? I have not played it yet)
-Mariokart:Double Dash (not so bad since it's sequel did have online)

I have read developers mention the fact that often online (multiplayer) development take away from the single player game since make a game run online isn't cheap (very time consuming). This is probably why we read that online play gets dump sometime during the development of a game.  For example recent Wii Worms.