I think a updated "Destroyer command" (way updated please) could be great. No need for multiplayer but multiplayer should be a choice for those PvP gamers.
Just like my whining about better hydrophones in SH5, I would really like a nice Anti-sub simulator with high fidelity hydrophones and sonar on the ship.
(yadda yadda yadda Platapus whining again about hydrophones. Sheesh he must have a good set of speakers he is proud of or something.)