Personally, the graphics of both SH3 and SH4 blew me away. For all I care, I would take either game's engine with minimal improvements for SH5. I would be more interested to see better AI, less bugs on release, wolfpacks, better crew management and sub upgrade interface etc.

I don't consider graphics to be the problem in the series, it's the bugs and that feeling of being alone while on patrol. The radio messages in SH4 didn't really cut it.

Remember that there are guys dedicated to each feature: AI, graphics, sound, gameplay. So the game evolves on all areas with every release. Can't evolve on just one of them (e.g. the gameplay). The reality is that the gameplay in a Silent Hunter game is more complex than all other areas but there haven't been enough people working on it.