Chris Paul, Dwight, Bynum, Al Jefferson to name a few free agents.
I don't really consider Chris Paul or Dwight Howard free agents since I don't think there is any chance either player leaves their respective teams.
Andrew Bynum is a very tempting name. A big man combination of Dirk and Bynum would be virtually unguardable. That would be an incredible sight. Huge injury risk and facing a $100 million five year deal. I don't know. Maybe.
Josh Smith is another big name. Smith, like A.Bynum, would take a king's ransom to sign though. Andre Iguodala I believe is free agent also but, like J.Smith, I would be worried about the cost of acquiring him.
I don't think Big Al will be worth pursuing. Too expensive. Very weak defensively. He and Dirk would work well offensively but they'd be far too weak on the other end of the floor. Bad fit. If the market fell through for him though, Big Al could be a good Kaman-esque signing like what Dallas did this summer with Kaman and OJ Mayo.
Millsap will be available too. Dallas seem committed to keeping Dirk at PF and slotting a center alongside him though. Limited value to Dallas.
There are a lot of good names there but I am not sure how much value there is for Dallas since they'd have to blow up their team and pay huge salaries for players will struggle to earn that paycheck.
Edit: I am looking up a free agent list now. A few other names that jump out = Brandon Jennings, Jose Calderon, Monta Ellis, Manu Ginobili
(like CP3 and Dwight, I can't see him leaving his team), Kevin Martin, Dallas' OJ Mayo, David West, Nikola Pekovic,
I think Brandon Jennings could be a good option for Dallas. Young point guard. Talented. Still has room for growth / improvement. He doesn't seem committed to Milwaukee nor they to him. Might be able to get him away on a solid contract.
Pekovic might be a good option also. Defense is an issue alongside Dirk but if they can keep the contract down it might be workable.
It'll be very interesting to see what Dallas does next.
They are in a strange place with Dirk and trying to compete for title. And the lack of elite talent around him. And trying to find ways to acquire top tier help for Dirk. You could see them going in a lot of different directions with that roster / cap flexibility.