Giannis signing now is the best case. Cuban would have waited around while other opportunities passed the Mavs by. Time to move on.
So with a projected starting lineup of Luka - Richardson - THJ - DFS - Porzingis, I think you could most easily upgrade THJ or DFS here. Ideally, I think having another off-ball wing that can generate their own shot would be nice. If you are trying to upgrade THJ then a guy I would be asking about is Zach Lavine. I think he is underrated and on a rebuilding Bulls team that might be looking for some assets / tanking. If you are trying to grab a more defensive-minded wing and upgrade DFS then I would be looking at Aaron Gordan. Similar to Lavine he is on a rebuilding team that might be interested in some assets / tanking.
As far as assets I think the Mavs are in a better spot than the last few years. I really don't think they have much in the way of bad contracts or bad players. I hate to say it but post-Achilles Powell is probably the most negative guy on their books at this point. So for total assets they have:
Draft: 2015, 2017 1st round picks, pick swaps, and 2nd round picks
Young players: Brunson, Bey, Green, Terry (hell, maybe even Nate Hinton)
Solid NBA players / Good value contracts: DFS, Maxi
Expiring contracts: THJ, James Johnson
I think there are quite a few options to be had here. I don't think any combination listed gets you into the Beal talks (top 20 guys), since you don't have a young star to trade out. However, some combination of this should get you into the conversation for top 50 guys like Lavine and Gordon. It will be interesting to see if the Mavs make an aggressive move near the deadline or punt to the offseason again. My guess is they are going to ride it out until at least the deadline, but I would bet a move is coming.