This championship D has two things going for it:
1. Two elite players, Vea and White, still on cheap contracts.
2. A upper-tier secondary, with good depth, still on cheap contracts.
Defense varies much more year-to-year, and can't be schemed up as much as offense. The Bucs need to maximize the current "cheap and elite defense" window, which will only last 2-3 more years. I think that means the focus of free agency should be defense. The first priority: Sign Lavonte David. Him and White together are the league's best LB duo, and they both limit opponents' run and over-the-middle passing games (especially if White's coverage skills improve).
Then make a good offer to Shaq Barrett but don't break the bank, because he is not elite and greatly benefits from Vea, as we saw this season. Other teams face even more of a cash crunch than the Bucs, and thus other decent pass rushers--who would also benefit from playing with Vea--will be available.
But signing these two should have locker-room culture benefits for the defense, I think. (Though I wonder if this whole notion is really true.)
After these two, the Bucs could swap pieces in and out without much change to the team, particularly on offense, where the return of OJ Howard, Gronk and Evans plus the O-line should make the offense as good as ever.
