Dallas' best shot to return to serious contention is going to come in the form of Bynum.
Aside from Dirk, Dallas has no valuable assets to trade except for 1st rounders that wouldn't be fully developed players until Nowitzki is done. Bynum is an X factor. I wouldn't ridicule front offices that wouldn't sign him because of all the baggage he carries and the price to be paid for such a risk, but he's the only player on the market that has the ability to be an elite player. With an insurance clause in the contract, Dallas should sign him.
A healthy Dirk and Bynum would be one of the best inside-outside, 4-5 tandems in the league. Add solid outside shooters or a player that can score effectively in isolation and they become a serious title contender.