He has to return for the 20, even if they carry him out afterwards! After this year, I thought they might go with HHH again, but I'm not sure now. There aren't many on the roster worthy of being his 20th to be honest. HHH/Kane have been done, Big Show wouldn't a great spectacle match, and Cena's tied up with Rock.
With the latest goings-on, Punk could be an interesting one.
Either way, it should be Taker's last match. He wins it and claims the 20-0 streak. Straight after the match, the whole arena goes dark. A single bolt of lightning strikes the ring, and when the lights come back up, Taker's gone. And that's it. Perfect ending to a perfect career.