I only bought it to play through the singleplayer campaign, don't play online at all, Nor can my computer run anything at all anyways right now, its on its last leg. I'm just glad I managed to finish Mask of the Betrayer before it croaked. I had some pretty crazy builds through the original campaign and MOTB though. I remember my character in the OC I was doing was a Drow, Level 3 Bard, 2 Fighter, 5 Red Dragon Disciple, 10 Arcane Archer which worked rather well except for that lack of good bows and inability to enchant bows to make them awesome like you can with melee weapons.
As far as more expansions, I don't know of any having been announced, but there is a small 'Adventure Pack' being released. A premium module being published by Atari called Mysteries of Westgate, no new races/classes or anything but its supposed to be another 15 hourish adventure to go through with full voice acting for the new characters in it. Not being made by Obsidian itself though.