M38 carbine
PPD-40 replacing AVT as 'super rare' Soviet weapon
DT-28
Early MG42
Luger
Maybe some other German-used pistols, like the PPK, VIS and Hi-Power. I don't see much need, though.
Molotovs and flamethrowers would be great, but only if the implementation was good (and they didn't end up bringing people's PCs to their knees).
I'm not a big fan of adding anything 'super rare' after seeing how the supposedly rare weapons went on release. The PPD could qualify, but unlike any of the 'rare' weapons seen ingame I actually do come across photos of those in use by both sides on at least a semi-regular basis (versus never for the MKb, AVT or MP 40-II...) so I think it'd be fine as a hero unlock for both sides or something. Really there is not a problem with lack of weapons right now; RO1 didn't have things like the Federov or SVT-38 or AVS-36, and it was just fine. Throwing more weapons at the game isn't going to make it any better, especially functionally identical weapons (basically all the pistols, the SVT-38 since durability issues aren't modelled in the game). You could bring this argument against the M38 to some extent, but making it handle a little nicer would kind of cancel that out (as would restricting it to non-rifleman classes who need a smaller rifle, say, engineers).