Combat particle effects!
Ok stay with me. At this point, the pacing is not brisk (though I have heard there will be options for that later) so this may turn out to be terrible. It would be great (terrible?) to have some explosion and smoke (bubbles since we're under water?) that billows and clears as the surviving sub pulls through the debri field.
Maybe even if it's only when using the time machine.
I know this isn't going to happen, but I really like particle effects.