Note: I am not a customizer, nor am I about to host a swap. I just don't think I've seen any swaps like these, and would love to see a bunch of different ponies with these themes (my favorite swap to look at was the monochrome swap, by far). Feel free to use these, or ignore them, alter them slightly, or whatever.
Rainbow swap - ponies with all colors of the rainbow! Could be rainbow striped or gradient bodies, wings, hair or anything else on the pony.
Metal/metallic swap - focus on (but not limited to) grays and browns and maybe golds. Everything has a metallic finish, and/or metal additions to the pony.
Inverted swap - either inverted colors of an existing pony (ex. hot pink Stockings with white hair, deep blue Glory with white hair and a purple stripe), or two ponies with colors inverted of each other (ex. a blue pony with red hair paired with a red pony with blue hair).
Historical swap - ponies donning historical garb from different periods and places. Could be human clothing or horse armor. Preferably no ponies painted as having human skin colors as their coat colors, because it looks weird.
Other animals swap - ponies having a cutie mark/full-body painting or decoration depicting another animal, as well as a custom piece of that animal to accompany the pony. Ex. a pony with seals painted on it might come with a customized LPS seal.
Space swap - has this been done before? I feel like it has. Title is self-explanatory: ponies have space designs, such as galaxies and nebulas.
Grayscale swap - the height of elegance! Ponies have colors ranging from black to white, and every shade of gray in between, but nothing else. Maybe not such a great idea for an entire swap, but I'm throwing it in here since it's my ~aesthetic~.
Feel free to add your own!