While the first two in Johnson and Joseph could be acquired for a later pick, Leonard Williams would probably take a third rounder, at the very least, to obtain. However, for a team like the Bucs who are attempting to get younger and more talented along the front four, Williams could be an option.
The difficult of making this trade work, however, is that Williams is entering the last year of his rookie contract, and without the assurance that he would re-sign and the cap space to do so, the team would have to franchise tag him after the 2019 season in order to ensure that he would remain a Buccaneer after 2019. However, With Desean Jackson, Demar Dotson, potentially Gerald McCoy and others on their way out, Tampa Bay is likely to clear at least an additional $30 million in cap space.
Williams is entering his fifth season in the league and is only 25 years old, so trading for him and locking him up long term would benefit any team that could acquire his services. For a team like the Buccaneers, who still believe their window to compete is open, a move like this could make sense if it only required a third round pick.