IIRC the idea behind the cruiser-submarine concept was not that they needed the big guns to sink merchantmen but to sink escorts. Escorts were usually not very heavily armed with guns as their main enemy was operating beneath the waves so depth-charges were more useful. The cruiser-submarine would sink those escorts which would force the enemy to use bigger and more heavily armed ships as escorts. Which would in turn weaken the main battle fleet.
Huh. I don't think I've ever seen that argument before. Thx.
Can I suggest one of the very early things to do is to expand steel production/steel industry, so limits on U-boat construction aren't hit as early, or boats built at the expense of something else really useful? (IDK how much expansion, if any, was possible...)
What lands might be open to German submarine designers after WWI, and before WWII? ... What about Turkey, Spain, Argentina?
I'd rule out the U.S., Japan, & Italy (tho the Italians could afford the help). Turkey, maybe. Spain IMO is borderline; she's never built anything so sophisticated, & is likely to be pretty broke thanks to CW. Argentina IMO is still too backward; building corvettes, yes, & maybe even DDs, but not subs, not yet.
develop viable submarine merchant shipping
They'd be too small until the advent of nuclear power. Even the
I-400s couldn't carry enough freight to be practical: bear in mind, a pretty small freighter carried about 2000 tons dwt, which is, what, double the displacement of
I-400? For practical transatlantic or transpacific, you want upwards of 5000 tons dwt.
The two most likely prospects for this that I can think of off the top of my head, would be the Walter submarines, or a submarine with enough battery capacity to power their engines for the entire time, and with a safety margin built in to avoid tragedy as part of such a requirement, this would have to be a very large capacity indeed.
Fuel cells were possible (they'd been conceived about 100yr before), but building them & getting them to deliver enough juice long enough isn't a trivial project. To do it, of course, you first need to throw Dönitz under a bus. (The same one you throw MacArthur under? Two for one special, today only.

)
This tech has implications beyond subs: seafloor habitats, for a start, & spacecraft...
For fighting boats, more battery capacity, more motor hp, & better hull streamlining would all be really good ideas. If the Germans just examined the airflow (hydrodynamic) characteristics of Zeppelins, they could've had boats that were
much faster for the same hp as an OTL Type VII or Type IX.

That is
bad news for Allied corvettes.
Let's also look at the weapons
I'm a particular fan of the pattern-running fish the Germans developed.
I also wonder if somebody couldn't have thought of *CAPTOR sooner, maybe even with a variety of *
Skvall.


For ASW, an ATW that could throw a DC would have been a great idea, except for one thing: would it risk sinking the ship firing it?

(When you run over the "explosion zone", do you get a "mining" effect?) Or could the charges be thrown in pairs, on both bows, rather than dead ahead?
Dipping sonar would be good, too. (Conceived at the end of WW1, never developed beyond crude hydrophone.)
I wonder if there was a prospect for using new materials to reduce the magnetic or sonar signature: plastics in the hull skins? Or
cermets?
And given the prevalence of Allied maritime patrol aircraft, what about a sub-launched *HARM?
Type IX rather than Type VII should be focused upon. Their longer range makes it necessary for RN to escort convoys and hunt submarines basically everywhere in the Atlantic, even in the Indian Ocean if supply is arranged.
Agreed. However, that sacrifices numbers, & again, Dönitz has to catch a bus.
Basically a torpedo with similar warhead, same machinery but less fuel? Additionally, could a heavier deck gun, perhaps around 5" be fit on Type IX for more efficiency in surface fights?
A shorter-range torpedo means you have to get closer before you shoot, which isn't always possible. The larger-caliber deck gun requires quite strong deck mountings, & more space to work it properly. Not to mention a sub being really vulnerable to counterbattery:

a single hit puncturing the pressure hull, & she's junk.
Now, if you want to increase firepower, adding a second 88mm...or even go up to a pair of 100mm/50cal...

And you detail them only for engaging solo merchants or polishing off cripples.
I like the idea of an anti-escort torpedo, a short-ranged homer. I also like the idea of sub-launched rockets for anti-escort work: something like 15cm
Nebelwerfer in WT cannisters on deck, fired from depths around 10-15m (periscope depth?).
Giving U-boats *Naxos from the outset would be good.
Not requiring them to send weather & position & ammo expenditure & toilet paper consumption reports on a regular basis would be a really, really good idea. (And, again, Dönitz & a bus.

I feel like I've walked into a Monty Python routine.

)
Any a/c or helo aboard, IMO, is a complete waste of effort. They take too damn long to unstow, assemble, disassemble, & stow; it can't be done submerged, putting the boat at tremendous risk while it's being done; & the benefit is marginal.
The Germans launched a rocket from a submerged tube and it allegedly was possible. BUT, here is the problem; it was a hot launch. Every missile firing sub that is submerged, that is worth a flip, uses a cold launch pop up ejection system with an independent gas generator to throw the missile clear of its tube for safety reasons. Some nations use the rocket motor ignition to generate the gases to pop the missile up out of its tube (Russians), but this has not been a very good idea for obvious (K19) reasons.
I see two options: deck cannisters (already mentioned) or a calcium permangenate "booster" (which wouldn't be a bad thing for deck-launched rockets, either).
Short flight rocket? Telemetry and launch problems.
Not seeing issues of telemetry with *
Unternebelwerfer. Launch was tested OTL, & (AFAIK) worked without undue difficulty. (I see nothing
here indicating it's impossible.)
solid fuel candle as big as a V-2
So far, nobody's suggested that; I'm certainly not: I'm thinking, adapt existing arty rockets.
And range needn't be an issue, if it's used solely against escorts that are closing fast...
Without nukes or sophisticated fuel cells, you can forget polar transits. Besides, there's nothing there worth shooting at.

You want to be off Halifax for days, & you want to transit the Atlantic in much less time without getting killed: turnaround time is key. So you want
faster, not more sub-Polar capability. Faster dived, for the times you
have to be, shortens transit time.
If we've got a clean sheet of paper (more/less)? *Type IX (longer) with more torpedo tubes (6-8 forward, 4 aft), more fish carried (30? More?), more streamlining & battery (for better dived speed & endurance), fiberglass conn skin, bow & stern sonar arrays, RWR (*Metox +P) on a retractable mast, 10x32cm deck rockets (WT tubes fold down into deck, for streamlining), 6x32cm *
Unterflakrakete (*Sub-HARM) in compartment aft conning tower, retractable radar mast, more horsepower from her diesels (5000-6000?)