Well, considering the Mavs don't have much cap space and no tradable draft picks, since the Knicks own most of them, the Mavs look to be stuck with what they largely currently have for this and next year at least.
Also, considering you didn't see high end free agents clamoring to play with the "nightly triple double" man in Russell Westbrook, there is a good chance that most high end free agents would not want to become an afterthought to the all mighty Luka and a 3rd fiddle to Luka and Porzingis in Dallas.
I would think most high end free agents would want to get their numbers too and I can see them wanting to touch the ball more than what a 3rd option would get in Dallas.
I doubt they get any big time player to come to Dallas through trade or free agency for the next couple of years. But, I could see someone like WCS signing to get some those lobs from Luka.