It's impossible to keep playing after the final boss is dead.
While I'm not sure if this is true for the latest non-beta version of the game (2014), in the beta I found a multiplayer bug that allowed me to load my final save and play - sort of. The host (who always plays the protagonist) gets booted to the opening menu (where you choose between the tutorial, Baldur's Gate, or Black Pits) after character arbitration, but the server is still running and the client can exit the temple, pick up and drop items, and fight any undead you might have missed on your way. Since the protagonist can't be moved however, that is about all that can be done.
Yes i know we can do that. but my current save is just in front of the final boss. So its lame to just "leave" and dont kill him. I know I can kill him and load, but its not the "same game".
Comments
I know I can just back up from the boss fight and return to baldurs gate. But hey, its kinda lame. :P
Why would you need to keep playing, when you can simply finish all quests before killing the boss?
WHATEVER. I'll go back... :P
http://forum.baldursgate.com/discussion/19439/mod-continue-playing-after-defeating-baldur-s-gate-enhanced-edition#latest
Its made by @Econon