On offense, the first player you must consider a building block is running back Joe Mixon. The Oklahoma product has had a down year. However, has flashed over the last two games after a change in some of the blocking schemes with the offensive line.
Mixon is still just 23 and has a lot of tread left on the tires. The coaching staff still needs to improve on involving him more in the passing game. He is certainly a keeper going forward.
As for other running backs, Giovani Bernard recently signed an extension and is also still young enough to keep around for a little while longer as a change of pace back.