Long answer: Fanmade palettes are not going to be included in the game because making a palette swap takes some degree of time, effort, and money, which Lab Zero has far better things to spend on than palette swaps. Back during the Indiegogo campaign, people that donated $400 or more could have any one character palette they wanted added to the game (Which is how we got palettes like Fionna Filia and Double Rainbow Double), but those days have gone by, so there's really no way to get fanmade palettes in the game.
However, I could be wrong about this, but I remember hearing once that Lab Zero does look at this thread for palette ideas on upcoming characters (e.g., if you made an Eliza palette now, they might look at it and consider it).
We still make palettes because we like to see how [x colors] would work on [y character], or because we feel like a [z character] palette would be a clever reference for [y character] to have.