Personally I'd like to see someone at the four who creates mismatches for Portland who can move down to the three if needed.
Although I've never been a huge fan of his, Gallo I think would fit perfectly with the 4 of Nurk/Lillard/CJ/Turner.
However with his PO, and the size of the contract he'll demand if he ops out. I don't think that's really an option.
More realistic are players like Amir Johnson, Quincy Acy, Trevor Booker, Thaddeus Young, DMo, Patterson.
Couple of those players would need to be traded for, but I don't think any of them are all that unattainable. OFC I'd prefer Patterson/Young to all of them. But beggars can't be choosers.