Quite a fun game!
I like the palette, the idea of fetching things with a bubble, the wizard's gestures guiding the bubble, and the ingredients of bat (familiar), hat (familiar, and interesting as an ingredient, and that particular hat looks unique), rabbit (interesting as an ingredient, and to have a dark one rather than the usual white rabbit), and donut (quite unique as a potion ingredient!). I like the jaws on the cauldron.
As to suggestions, I notice that if you press a direction too late then it applies to the next bubble, usually crashing it. A delay of less than a second before a new bubble appears or is mobile should fix that (but a second or more would probably add too much dead time to the game).
You could make it so that the recipe only shows when you first reach a level, so that you don't have to press C twice to restart the level, since after all you can always press C if you want to check the recipe. I notice that X doesn't actually do anything in this version.
For balancing the difficulty I don't think slowing down the bubble would be good (except maybe very slightly) since you don't want to make the beginning levels too easy or add time to the game, making it feel tedious. You could probably leave the number of bubbles the same throughout the game, since the challenge increases enough by itself, and the main thing that adds tedium rather than difficulty is having to redo something you've already accomplished.
Actually, apart from the decrease of bubbles, I think the challenges increase in difficulty pretty nicely, and seven stages is a good number for the difficulty (and artistically appropriate for a small game about a wizard).
As to artistic suggestions, I would say to make the moving blocks look different and strange, since they are an addition to the last levels which make them unique and change the way you play. It's always nice to reward success with some artistic discovery. I'm curious what kind of end screen you'll put together for it - I got past stage 7 :).
I appreciate that you have a Linux version download too!
Thanks for a fun challenge!