To give my opinion about minor nations in the game, they are already more than overpowered in Vanilla compared to Majors Nations.
According to the economic historian Kennedy, the 7 majors nations alone in 1936 had more than 90,5% of the world economic war potential (in game, all 3 type of factories).
If the Vanila game was more realistic, there would be no point at all to play minors outside of multiplayer games.
HOI4 is not Europa Universalis. You don't have centuries to turn shitholes into world powers. HOI4 timeframe is only a bit more than a decade, so we should not expect any change in term of Great Powers. In History, Canada was the only "minor" which achieved to economicaly outproduce a major (Italy), but had already the potential (technologies, car industries etc) to do it.
Guess what? The 1936 "majors" are still very powerfull nations, part of the "G7" (same Great powers than in 1936 with Soviet Union replaced by Canada), and it took a few decades for China to be effectively a Great Power (and India is in the way).
My conclusion : it is pretty WAD that minor nations are boring, and can hardly do anything relevant in HOI4. The best thing a minor nation could do against a major is to defend heroicaly a few months like Finland, but should never been able to invade and defeat a major by itself.