Why would a marine be a ''villain'' for Luffy to beat? The marines aren't suppose to be villains. It doesn't make sense if you are not a marine fan who wants his idol to be the last strongest villain for no reason.
Luffy fights with pirates, and maybe world government if somehow changes his goal, becomes a revolutionary leader, and decides that he need to beat Celestial Dragons and Imu. But that's it, Admirals can never be a final big bad boss, because there will always someone who will be another Admiral; Aokiji and Sengoku left; Green Bull and Fujitora came. It will continue like that. The marines will always be exist.
But if you remove Imu or Yonko, then you can't find people who replace their powers and influential.
Even in a scenario, where Luffy quits being a pirate and replaces Dragon as the revolutionary leader, he would fight with Celestial Dragons and Imu, beating the marines won't solve anything.
But why would people accept a former pirate as a the revolutionary leader? It doesn't make sense, Luffy should always remain as a pirate.