Knowing the Sopranos and seeing how the writers often seem to build the viewers up for action and then let them down, I have to agree completely with TSoprano here and opt for the "Godfather II"-like ending, in which Tony probably whacks out someone major, and then we just fade away from him, never really finding out what happens to him afterwards, we're just supposed to assume he went on living his life.

Sopranos has always seemed to conciously try to avoid being cliched, and I feel as though going with any of the traditional and much-discussed options (Tony rats, Tony gets whacked, etc.) would just be too easy for them. They have to torture us.