Alright, so I'm currently around the 15:30 mark of the video, and I can already tell you one thing; He will be similar to Udyr when it comes to his relevance in a Meta. What I mean by this, is that one of the four options will ALWAYS be a better choice than the others, simply due to item path, runes, or other relevant champions. So while you can technically go into any one of the four forms, there will always be one supreme form.
As for the WAY you get these forms, I find a bit annoying. You basically have to decide right at the game's start, "I'm going to completely commit to this playstyle". And while you may argue that that's what people do when they select any champion, there's a difference. People are able to choose a champion based on their laner, AP/AD ratio on both teams, etc. With this champion, I feel that he will simply be chosen in the IDEA of variance, but will then be set-in-stone as to what he's doing the rest of the game.
I will say I like how each form has it's own form of "scaling" however I feel that Great Flame is out of place. While Mana is definitely an essential tool for a mage, I feel that it just doesn't fit. That alone makes me not want to select Great Flame over the others, simply because Mana becomes a very irrelevant (obviously not literally) resource the later the game proceeds. So having mana scaling feels like a waste, in my opinion. I feel like either Health, Attack Speed, or AP would simply be a better choice.
I'm not going to critique every single ability, simply because it would take me years. But what I will say is that, while definitely unique and interesting and well-thought out, having a champion who has 16 possible abilities is just unhealthy gameplay, purely because it would require a lot of knowledge on the champion to understand what is even going on.
I like the concept, but the execution is simply too much to keep track of, and quite frankly it's just four champions shoved into one.