With SS5 comming out I hope that the NF development team had paid attention to my SS suggestion on the old forum
Also I think that Japan instead of tier 5 and 6 subs they instead get to chose special projects which at the moment include the I-201 Sen Taka desiel-electric sub and the I-400 Sen Toku  carrier sub

SS5:
US: Narwhal (4 forward torpedo tubes, 2 rear torpedo tubes, 1 5" gun) DP: 21000
UK: Trition (4 forward torpedo tubes, 2 rear torpedo tubes, 1 4" gun) DP: 19600
KM: Type IXC (4 forwar torpedo Tubes, 2 rear torpedo tubes, 1 4.1" gun) DP: 20200
MN: L'Aurore (4 forward torpedo tube, 2 aft torpedo tubes, one 4" gun) DP: 20400
SN: Project 97 (improved S class) (4 forward torpedo tube, 2 aft torpedo tubes, one 4" gun) DP: 20000

SS6:
US: Gato (6 forward torpedos, 4 aft torpedos, 1 5"gun) DP: 23800
UK: Amphion (6 forward torpedos, 4 aft torpedos, 1 4" gun) DP: 21500
KM: Type XXI (sorry no snorkel) (6 forward torpedos, 4 aft torpedos (note the Real type XXI had less torpedo tubes and no guns), 1 4" gun) DP: 22400
MN: Surcouf (8 forward torpedos, 4 aft torpedos (I dont really know who the Surcouf's tubes were laid out), 2 8" guns(the guns would have less damage bonus than the smaller deck guns of other subs) DP: 23600, Note: The Surcouf takes longer to dive than most other submarines with the only exception being the I-400
SN: K class (6 forward torpedo tubes, 2 aft tubes, 2 4" guns)

Special Japanese Subs. the Special Japanes Subs will have special ability
I-201 Sen Taka (4 forward torpedo tubes) Special Abilities: able to go a fast as 35 km/h underwater and 29/km surfaced. Has snorkel which can be turned on so that the sub can recharge while under water but this makes the sub easier to detect.) DP: 22800
I-400 Sen Toku (8 forward torpedo tubes, 1 5.51" gun) Airspace/(Flight Deck): 1250/ (3). Special Abilities when on the surface the I-400 can act as an Oyodo and can launch scouts and torpedo bombers. The i-400 is slower to crash dive than other subs. DP: 24200

I am thinking about a modified HEI-1 that can launch Type A Ko-hyoteki midget. These midget subs act as underwater torpedo bombers that can launch up to two torpedo then they have go back to the mother submarine to reload. These midget subs have a dp of 100 so they are very fragile a single hit would sink them.