Just finished formatting that first draft of the Arcanist. I'm not sure about it in some ways - it feels like I'm not really using spell dice differently from spell slots, except as a mechanic to keep a 50% chance of getting back a lower level spell after casting alongside a decreasing chance of keeping the same slot. I was originally thinking about having the result of the spell die roll also have some effect on the spell cast (like changing damage type, messing with the number of targets, etc) but I couldn't come up with anything that wouldn't feel off in some way (possibly because I tend to dislike random mechanics).
It's probably better than my first thought, though, which was to have an increasingly large pool of d6s - at the beginning of each turn, you rolled all of them, and the total number of 5s and 6s you got was the level of spell you could cast that turn, and you could expend a die until your next long rest in order to increase your allowed spell level by 1 (but of course, each time you did that, your max and average would go down until then). That would have been total chaos.