I've kicked around a few ideas for aesthetically-curated cubes here and there, and I think it's a lot of fun to flip your usual card choice process on it's head. Feels weird and cool to look at the art first before you even begin to look at the textbox or the stats, and it is immensely satisfying to look at the visual spoiler on cube cobra for a cube that's curated for art and aesthetics (
just look at all these cute fuckin guys). The absurd amount of art tags on Scryfall also helps a lot for working on these kinds of cubes.
Two of the biggest issues I've run into while trying to work on stuff like this:
-Balance: When you're aiming for aesthetic choices above everything else, it becomes very difficult to balance your card pool. Often times I'll look at a card for that pets cube and be like omg its perfect look at this sweet little man but the card just sucks shit on rate and it would be hard to justify putting it in any cube. And the opposite, cards that have perfect aesthetics, but are just way outside the range of what the rest of the cube is doing. I think you can solve this by introducing tons of customs and modifications to make the cards more in line with each other, but that's a lot of work and does somewhat dampen the spirit of trying to curate a list of real cards that fit in an aesthetic box.
-Narrow cards: A lot of aesthetically perfect cards just won't work without the proper support. I think this can be alleviated somewhat with (again) customs, and also maybe allowing the cards to guide you toward specific themes, but there's always going to be stuff that FEELS perfect but just won't work.
I think your best bet for actually producing a functional cube out of a concept like this is to try to keep your theme as broad as possible, so that you have a wider card pool to work with. I would definitely love to see more folks pursuing these ideas!
And of course I love the Gay Cube