He doesn't need a whole lot changed in my opinion. He is extremely due for a new passive, I like the idea of that one as it reinforces the idea of his bruiser playstyle as you will never get to truly utilize it if you go assassin. While it's better than stone skin to some effect, it's still lack luster in the sense that this should be Wu Kong, the same one of legend. The idea of him getting stronger as he gets more beaten up isn't entirely alike the concept.
While his entire kit isn't a pinpoint representation either, I feel that the passive in particular could become something more meaningful.
There are many things that he did or had associated with him in his legend, while I am no scholar of him or anything. Much like stone skin, he did receive an armor from the Dragon of the East, and other tools along with other stuff. It could be easy to draw from something like that, or something more particular to his natural abilities.
I like the idea of stone skin playing into the fact that it is very very hard to try and kill the monkey, but to be honest it barely even feels like it. He was witty, and while his decoy could prove a very good substitute as moving it to his passive. I don't like the idea of doing that, stealth and the ability to dodge a hit or skill all together doesn't feel good from the other players perspective. If it were to be his passive, there'd have to be terms to where it can be activated, but not something expendable. It'd have to be baitable and such, last thing I think we'd want is another malzahar passive. Well then again, maybe it'd fit the monkey king more than malzahar as a melee, but that's entirely subjective.
I love the Q as you wrote it, and myself also think that it should have been like this long ago. In the same sense that it could also be used as a auto-reset if not charged, as the comment suggested, similar to pyke and his hook.
I personally felt that the monkey needs just a bit more utility, and something that is more along the lines of a higher skill ceiling. I figured his W could be tied to his ult, much like the clone but instead the clone will knock up what is around it as well, but only on ult activation. There is very little punishment for interacting with it and most players now will be able to tell it's not him when it's just sitting there. While it is good for mind games, and baiting abilities, it needs to have some form of penalty as the damage it does feels laughable and entirely avoidable. Considering the cooldown, I was never opposed to having it have a knock-back in particular, a brief one on par with its radius. I don't know about reactivating the W though, it might make it too easy. So you need those 200IQ strats, it'd function more as a choke and make it harder to do things like dive the monkey. Which is something I feel should be on the don't do list.
His E is fine as it is, it's pretty fast too and has a certain element of surprise that goes well with his current W. I wouldn't touch it, also the attack speed is nice but the purpose is a bit odd.
His ult is hard to change as it does its job. It's a bit weird on terms of damage, it applies cleaver very well, but the damage of his ult can feel slow. The mobile AoE knockup is the most notable part of it. If it had a passive linked to his Q, i feel this is where the idea of adding extra range would fit well, not to his ult but to his basic attacks and/or Q. He doesn't have natural sustain like many other top laners, and while he has other tools for survival, it's clearly not enough. He'd still be melee, but much like other champions who could use the literal range of their weapon.
Also his Q again, what I'm getting is that riot didn't think that the monkey king with his extending staff should be able to poke with it. It gets pretty big, and SHOULD BE the most fundamental part and recognizable part of his kit, but of course. That's just my opinion.
tl;dr
go ahead and skip it if you don't want a long read.