Having a map options, like having a abyss option (like if you were outside the map; no spot to land)
Another idea is having a mission were 2 subs compete to get a artifact, and the sub to get it to there base first wins

What do you mean with abyss?

I know what abyss is, but what do you want from Regalis to do with this?

And about artifact I believe we can call your game mode idea: "Piracy", where as you said 2 submarines will have not only to kill each other, but also to get same artifact.

I meant abyss as a example, but having options like a slider's for different gameplay changes (for example, how much terrain, or how many creatures in percentage)