Wario Land II was a transition point from the 1st Wario Land. It introduces the main physics, controls, & moves that define Wario Land for most people: Wario’s invincibility; a greater focus on puzzle platforming than action, which better suits Wario’s still-floaty — but not as floaty as the 1st Wario Land’s — physics; & most importantly, introduces possibly the most famous twist o’ Wario Land games, the status effects where Wario gains special power by being hit by enemies, which in different contexts can be helpful or necessary for reaching places or stifle progress, whether it be springy or puffy Wario making Wario move higher than his jumps can move him, whether he wants it or not, or flat or fat Wario, the former o’ whom can go wider distances & go o’er gaps in thin passageways, the latter o’ whom can break thru ground that regular Wario can’t, but both o’ which have weaker jumps & cannot enter doors.
That said, I don’t think Wario Land II realized the true potential o’ the series, & this mainly comes down to its levels: with the exception o’ a few branching paths to multiple endings, which are basically glorified secret exits like those in Super Mario World, & the treasure doors with their repetitive, tedious minigames you have to find in e’ery level, as well as the also-tedious & repetitive guess-the-# game, which are like the bonus coins in Donkey Kong Country games, but with e’en mo’ repetitive challenges, this game’s levels are still linear &, due to Wario’s invincibility, don’t offer much in terms o’ challenge.