As you need both graphics and gameplay (you can't go back to SH1 graphics) I think bang for the buck has to be looked at for where time is spent.
Things that get used all the time should have top priority. So for graphics, things like water, weather, main interior, enemy ships etc. should be done. For gameplay, AI, damage model, campaign etc. should be done. All that should have priority as it's in use all the time.
Then there's lots of graphic and gameplay extra that are nice to have but might come second. I'd personally like a full interior sub (doesn't need to have crewmen) and visable damage and flooding (not just that one leaky pipe), but it's all second to getting a proper damage model in the first place (no one min repairs etc.). I've seen some gameplay talk of sub special landing missions and sub vs sub battles... but how often is that going to happen, imo getting a good campaign working is more important or even my interior damaged graphics. So that means that minor gameplay stuff will come second to major graphics and major gameplay stuff comes over minor graphics. Now what YOU think of as major and minor (graphics/gameplay) depends on your gaming tastes, but I'm more talking about how much each has an impact, how often you are going to see or use that function.
Just my 2c
T.