It all comes down the the Twins evaluation of Sano. If they're confident he can play 3B adequately then they should shop Plouffe. If they aren't then Plouffe sticks around another year and Sano fights for playing time with Mauer, Vargas and Arcia. I think they'll have more suitors in the offseason than at next season's trade deadline. Trade deadline deals are typically for impending free agents.
I don't think a utility role for Plouffe makes sense for reasons hinted at above. It is harder to learn 4 positions and play them well than to learn one position. Plouffe has shown he can handle 3B and be an adequate starter. Someone will value that higher than a utility guy at the corners. Turning Plouffe into a utility guy damages his value if it hurts his 3B defense.
Unlike CF where the Twins had no acceptable backup plan when they dealt Revere and Span they do have decent stopgaps in Escobar and Nunez and an additional backup in Deibinson Romero.