Ray Allen would be great, as others have said would complement our existing pieces, spreading the floor for Melo, Lin (assuming he signs) and Shump (when healthy).
I would take a look at Tracy McGrady for the Vet Min. Feasibly could backup 3 positions (PG, SG, SF) and would add a veteran presence coming off the bench. There was talk of him signing for less money to come off the bench when we were going after LeBron/Wade/AN Other max contract in 2010 and although age and injuries have taken their toll, maybe a 1 or 2 year Vet Min deal could be of some benefit, especially if (and correct me if I'm wrong, I'm not totally up to speed with cap restrictions) signing for the Vet Min wouldn't affect our cap space/MLE etc too much if at all.
This roster, if possible (like I said, I am far from familiar with cap limitations so this might be totally impossible), would IMO put us in a great position to challenge for a title.
PG: Lin, Nash/Kidd (MLE), Douglas
SG: Fields, Shump, Allen, JR
SF: Melo, McGrady, Novak
PF: Amar'e, Jorts, Odom?? (Bi-Annual exception?? Do we have this? Will Dallas waive/cut him? Lots to think about here)
C: Tyson, Jordan
Plus any draft picks we add...