My take as a player (and ex USN submarine officer if that matters)
1. I think the market is only saturated in WW2 subsims. I think that a spiritual to Dangerous Waters is long overdue. I'm loving Cold Waters but a real modern sub SIM would be wonderful.
2. If you agree with #1 then make it moddable. Greatly extends life.
3. Read or ask real submariners in your nation state(s) of residence about modern submarine operations...most modern subsims/games really miss out cool stuff that matters and is "fun" for a certain kind of player. Best recent example I can think of are both in Cold Waters. Game doesn't model diesel electric propulsion because it doesn't matter in combat. True. But it really matters on the campaign layer and I WISH they modelled it because it makes their 1968 campaign way less historically interesting than it should be.
4. See if there's a way to actually do raytracing in your sonar modelling. I don't think that any modern subsim has ever done this and it really matters. I think they just use standard formulae from Ulrich et al.
5. FOR THE LOVE OF PETE WHEN YOU MAKE YOUR SCENARIOS PUT IN A REALISTIC NUMBER OF BIOLOGICS AND MERCHANTS! I should be contact saturated in any littoral area in the world...even in 1968.
5. Don't expect to make a ton of money
v/r
feld