Hello,
there are a lot of subs they could bring to the market...
VII A, II D, VII F, IX A, IX D1, IX D2 all the Walter Types and so on...
But let us be realistic: They never will make a sub only for some enthusiasts. They want to sell a lot.
In this case the Type XXI is one of the best choises. Like done with the VII and the IX they can sell variants:
XXI (WWII), Wilhelm Bauer, U-2513 and U-3008 (at US-Navy), U 2518 (French "Roland Morillot").
Some years ago there was a private seller of 1/72 scale resin subs. There i saw also a Type XB. Great build but very expensive.
All the other types we had to build by ourself...
...or we find someone who want to start a small series production...
To the scale: I prefer 1/72. Large enought for a lot of details.
At 1/144 you will find a lot of diferent types, but in some cases they are nearer to phantasie than a real scale model.
I started at this scale.
I also "tested" 1/35 but therefor i have not enought space at my home...
Regards,
Uboatfan