You take SH4 and put modern submarines into it, modern weapon systems and modern asw warfare. Base it on a full scale conflict.
no more fannying about on little 500x500 nm maps
no more play-the-cop-of-the-world-catch-a-drug-baron-or-terrorist-in-a-midget-sub-before-he-blows-up-the-world-clap-trap-crap
no more canned missions whose only dynamic elements are does the akula start in the northern or north eastern coner this time?
no more start the mission beamed straight from your base and into the thick of it
no more annoying briefings "you have been hunted by the bad guys' toughest asw forces for days and are now put straight under 50 of his best krivaks and in between 25 of his elite attack subs, escape without being detected or the mission will end with a female voice saying game over" ....let the mission write itself with proper ingress and egress from the base
And it tastes great too

yum yum