You're certainly right that only advantage items would make the game too easy, right now it's hard but not unwinable (is that a word?) it's just that I've played it quite a bit and without having a lot more or more interesting items there's not much replayability. Also, are you going to make starting shapes? Like if you kill 500 triangles you get a fast character with low health, or killing bombs gives you high health but only contact damage? Adding unlocks would massively increase replayability. Again, I do love the game and I've been playing it for months now, please don't massively change it, aesthetic and core gameplay are very well-made.
The core gameplay hasn't changed a lot, if at all. It's just polishing that was added, a few new items, and performance improvements.
The development is nowhere close to the end, and the game can still be called a "prototype". It will definitely receive maaany updates regarding items (my plan is having at least 50 items), enemies, bosses, performance, better UI and even replayability.