Currently FOTRSU 1.1 is the best mod around for SH4, it has the best interiors and the widest camera angles for a true immersive experience. Then, a lot of new ships added.
I would add an optional mod for it to make contacts all grey giving you a harder game - but please: only optional! That's about it, I have no other wishes, the mod is perfect if more traffic is added into its campaign, also with the new units, but the team is working on the mod, so there is hope
Alternative good mods are all those which allow free camera movement and clickable items inside the submarine:
TMO 2.5 + OTC
Ralles ModSoup (TMO + OTC + RSRDC)
RFB 2.0 (for those who like realism)
Then...There are mods to have playable ships in the game, but they all need a dedicated campaign, but otherwise they are very eye-candy and fun to play.
XS