At least one of Tyson Chandler and Taj Gibson should be on the move this off-season. The Mavs just need to acquire one of them. They're in a similar position as the Spurs are with Duncan; it's just that the Spurs have three extremely talented defenders to help Duncan out (and Duncan can protect the rim at an elite level if he can play in a phonebooth). Cuban needs to focus on acquiring defenders to put around Ellis and Dirk.
Ideal off-season for Dallas: re-sign Dirk for cheapish, trade for Gibson, sign Deng, sign the best room-exception player they can and draft the best scorer at 34. They could also try to trade for Tony Allen and sign Ed Davis and take a risk on Granger on a near-MLE deal.