That's it - once the final fight is over, the game ends. You can only start a new game. You could also get ToB and either replay the final battle or import your party from your last save, and carry on from there.
Note however that you can't return to Amn to finish your outstanding quests. To do that you'll have to load an earlier save (before fighting Irenicus in the Elven city).
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"Butt-kicking for goodness!" - Minsc
"Cities always teem with evil and decay. Let's give it a good shake and SEE WHAT FALLS OUT!!" - Minsc
