It depends a lot on the game mechanics. From a plausibility perspective, even China wasn't likely to become a full-fledged major in the game's time period, and none of the secondary powers had buckley's chance. It looks, though (just looks, mind, I've only seen what others have), that in-game the ratio of occupation cost : occupation benefit is a good deal more gentle than historical, so it's just a question of time and potential expansion for the starting power (ie, belgium probably isn't in a great spot, as there's not much to expand into) and whether the other majors will intervene or not (which, again, they should). My preference is that it'd be a shame if HoI4 was easy to blob in a la EU4, but that's just my preference, and I'm happy to mod some challenge in for playing as a secondary power if it's not there in the base game (as if a secondary power can regularly be made into a major power within HoI4's timeframe, on normal difficulty, then the game's going to be lacking on the challenge front).