From what I can tell, nothing. After you beat the main story line, you're locked into episodes. You don't get your team back at any point, and there is no reason to do them other than to put warlords into your gallery, and the only reason to do that I can imagine is that maybe multiplayer battle lets you pick from your gallery?
I honestly don't know, but I feel incredibly ripped off at how the after-game was handled.
after you complete the main story (nothing more than a long, easy tutorial) you unlock the episodes
after you complete all the episodes, you will unlock the final episode named something like 'hero mode'
hero mode is basicly the main story, only the AI gets turned on (they will attack, get more warriors, build up cities..) and will allow you to work on the 'real' ending, i guess.
weather or not that makes it worth all the replay just to get to it is up to you --- PSN ID: Riftweaver81 I cannot be caged. I cannot be controlled. Understand this as you die, ever pathetic, ever fools-Jon Irenicus BG2