You can't go back to the acts, sadly, no...
It would be great to be able to Riftrun back to clear up any missing quests you may have missed the first time through...
***
I just tested the end Battlefields, and they are a cakewalk, with no challenge whtsoever. Nothing is taking more than one hit to go down.
Also, the Minimap is disabled, making navigation very awkward, and there is no easy way back to town when a dungeon is cleared.
I really would NOT recommend playing beyond the final act of the game itself.
