It will have to be someone really good on a team that isn't good enough going forward to win the championship or the team is financially tied up. So the candidates are:
LaMarcus Aldridge
Gordon Haywood
DeAndre Jordan
DeMarcus Cousins
Kawai Leonard
DeMar DeRozan
Greg Monroe
None of them are super realistic, but there is a chance for any of them. I would give a huge offer to Kawai, although the Spurs will almost certainly match anything we can give. I would give a huge offer to Monroe, but he will probably go to a bigger market.
I would also like to keep our draft pick if it's top 10, unless we get a really really tantalizing offer. Everything is on the table, but we'd have to do it for someone really special. Getting a good player on a rookie deal keeps us really flexible going forward. I would probably trade it + Sully + Wallace for any of those guys above, though.
If we could get Kawai and Monroe and keep our pick I'd be elated, although come on that's not happening.
I ultimately think that we're going to have to swing a mega-trade for a disgruntled super star again. I just don't think we're going to get a superstar via FA and drafting them takes forever.