Chopper is the key to luffys next upgrade?!
This 'upgrade' isn't haki or anything, which is obviously still something luffy needs to work on, but it's an upgrade to his devil fruit. What do I mean by this? Rumble balls!
Chopper had 3 forms before he had any rumble balls (reindeer form = walk point, hybrid form = brain point, and human like one = heavy point), and once he takes a rumble ball, he gains access to 4 NEW temporary forms (arm point, guard point, horn point, and jumping point). This is proof that a zoan will gain access to some new type of forms when they take a rumble ball.
We know that chopper is able to make a different variant of the balls, which we have seen when Bepo from Laws crew took one to activate his Sulong form since he is a mink (and he nearly solo's the whole crew). This proves that chopper is able to modify these to cater to different situations, where it can affect a mink with no zoan devil fruit.
Chopper is also known to be super duper smart. Evidence - Queens 'masterpiece virus' the ice oni he used during onigashima, which I assume means he worked quite hard and long on it, and is confident that nobody except for him can make a virus. And yet, chopper was able to make a cure for his masterpiece within one hour, in a battlefield. Imagine what he could do given weeks/months to study luffys devil fruit, so that he can make a special rumble ball just for him. Plus now with the Vegapunk connection they have, it could be even quicker/better.
Some drawbacks I will admit: could just be an animal thing, maybe it only works with his devil fruit since he can feel it's wavelength better, mythical zoans mightn't be applicable for this (especially a human form zoan), and all the other drawbacks you would have thought while reading this :)
Let me know what you guys think! I thought of this around when it was revealed luffy has a mythical zoan, and I couldnt see anyone saying this kinda theory anywhere either