There is no single move this team can make over the summer that makes them anything more than a playoff/playin team. Now is not the time to trade a frp + vet salary for a better vet. They have to keep drafting talent and looking to acquire as many picks as possible and avoid the type of move that negates what they've started building.
Look where OKC is today. They got there by getting as many picks as they could, then building through the draft. It's going to take several years to get to that point and trying to speed up the process will only set them back.
Get rid of vets for as much draft value as possible and stay the course. Anything else will just lead to another rebuild sooner than later.