1) A complete Royal Navy
Or as complete as can be attempted. In 1939 the RN was still the biggest fleet in the world. It'd be more than a little disappointing if, like SH3, SH5 out the box consisted of a Revenge class, a carrier, some destroyers and a corvette. The fact that the
Barham subclass,
Ark Royal and HMS
Courageous - all sunk by U-boats! - were missing is unforgivable. The RN is, after all, very much your main enemy in the game, just as the Kriegsmarine considered it during the war.
2) Randomised AI behaviour
One of the reasons why I've had diminishing returns with SH3, even after GWX, is that the actions of AI units are so easy to predict after a few months of playing.
3) Ahead-throwing weapons!
Most importantly Squid and Double Squid, with
Loch Class frigates to carry them. Also, Mousetrap for US ships. Maybe even Shark - if it's valid to play as a Type XXI, it's surely valid to get fired at with experimental Allied weapons!
Squid is most important, though. It's not like it was an obscure weapon - it accounted for dozens of U-boats.
4) Realistic damage modelling and graphics based on real war accounts and photographs
Every individual ship should have unique compartment modelling to allow for different damage behaviour every time a torpedo hits.
That means:
No more putting one eel in the bow of a ship with a sinking guaranteed as the captain drives his ship underwater
Ships can stop or slow down to attempt damage control, even if this is just HP restoration or pumping out
All ships can break up, with either section sinking or staying afloat. Ships can lose their bow or stern sections and remain afloat. Warships can capsize, as they do in GWX
5) A full, proper and unified physics engine
6) Don't expect or rely upon modders to fix bugs, playtest or fill in the gaps for you!
7) Accurate ship behaviour
No more F1-style acceleration and stopping from 20yr old merchants and warships. Realistic turning circles and tactical diameters.
8) Remember your constituency - given a choice, realism and historical accuracy are far preferable to fancy graphics and effects
One last thing: I have to disagree with people asking for playable surface ships. If you attempt that, the sub portion of the game is inevitably going to suffer. Likewise, a combined Atlantic and Pacific game with the option to play as German, US, British, Japanese, Dutch, Siamese, Free French, Albanian, etc. is also going to have the effect of watering down the core experience. Pick one nationality - my choice is German - and stick with it.