I like the idea of giving Germany rockets, I’m not sure how much it will do for balancing but, combined with a SBR or two, you can certainly put a hurt on Rus or UK’s income, esp if you do well in Afr.
3rd edition rules are good, but no teching doesn’t really do much, as it often comes late in the game, so even with rockets you still might need a little balancing. You mentioned (in you other post) that the players are about even, so I assume it straight up games Allies win 90-95% of the time. I would suggest still playing with a small bid even with rockets. Maybe 6 IPC for Ger (2 extra inf in Lib). The Allies might feel they are at a disadvantage, but it certainly would be a challenge for both sides.
Submersible subs, doesn’t favor anyone IMO, it’s just nice that they are not necessarily sitting ducks.
In my opinion 2-hit BB favor the allies, it often costs Ger an extra ftr, maybe 2, to take out the British ships. Too costly for Ger. Like BB said, Jap usually owns the Pacific anyway.