Here is my take, from Koko, obviously
During the late 20's as part of their submarine buildup project, Koko no Kaigun came up requesting a design for a large ocean-going cruiser submarine to supplement the small and medium boats being built and designed during the decade. Requirements called for long range, heavy armament and spotting capabilities optimized for SubRon leader duties and prolonged solitary deployments. The resulting design was the largest submarine ever put into paper for Koko no Kaigun up to that point, and would remain so for over a decade.
As planned the boat would have been 109,9m long, 9,7m abeam and with a surfaced draft of 5,9m. All for a displacement of 2.847t surfaced and 4.214 submerged. Poweplant was intended to be two diesel-electric motors capable of generatin 8.600shp surfaced and 3.000 submerged, allowing for a top speed of 18knots above the surface and 8knots below. Fuel bunkers allowed for an endurance of 17.500 nautical miles (32.410km) at 10knots, while on batteries they could travel submerged for 70nautical miles (130km) at 3knots. Some of the ballast tanks could have also been double-used as extra fuel bunkers, extending the range to 21.000 nautical miles (38,892km) if ever needed. Design depth was 140m, with test depth set at 80m. Armament wise, the submarines would have been fitted with two 120mm/45 Type11 guns in single mounts, one for and one aft of the sail, plus a single 13mm/76 Type93 machine gun. A total of eight 533mm torpedo tubes were also planned, six at the bow and two at the stern, with a full complement of 24 warheads carried. Finally, aircraft facilities with a catapult and a small hangar to carry and operate a single Yokosuka E6Y1 floatplane were to occupy most of the quarterdeck. 86 men (8 officers plus 78 enlisted) would have manned the submarine, plus two mechanics, one pilot and one obesrver to man the reconnaissance plane and aircraft facilities, for a grand total of 90.
The design was finally approved in late 1929, with the order placed for at least two boats (with Koko no Kaigun planning to order more if the sea trails would have resulted successful), to be laid down by the end of 1930.
The project was eventually killed by the ratification of the London Naval Treaty, which put the entire Koko no Kaigun submarine buildup program to a complete stop for more than three years (construction of large-sized units wouldn't even resume until five).