Hmmm...
Being interested in proper simulations my wishlist for SH5 would be as follows:
1) Independent control of fore and aft dive planes, individual powerplants and dive & trim tanks, including being able to use electric engines when surfaced.
2) Make the crew a bit less retarded so that they f.ex. don't report every friendly unit they see, or endlessy parrot "ship spotted" when they see a convoy.
3) Make the game a bit more scalable for people with older computers. This could f.ex. include an installation option where one could only use 256x256 textures if one wanted.
PS. Merry Xmas all