Even if it's late in the series (like the excellent Darth Maul vs. Palpatine duel) it would be a thrilling end to Ahsoka's journey. Plus, maybe (unlike Obi Wan in ep.IV) Ahsoka pounds the crap out of him and almost kills him as Vader - since he's in the armor who knows how much more punishment he could take?
I would love for this fight to be the finale of the series. I'm also hoping they incorporate the fanwank idea I had for the finale of TCW and have Ahsoka pop up as Leia's nanny. It would only make sense that she would have a guardian angel like Luke did.