I've actually designed forum skins before for other forums, one of them in particular being a Pokémon one (Pokémon Elite 2000). What I did with these skins was pretty different from what Serebii does with types and contest types, but I figured I could throw the ideas for themes I did for that forum on the table and see what you guys think of them. Unlike some of the other suggestions, these themes aren't Pokémon-specific and could be blended with just about anything, but I feel combining them with Pokémon actually turned out pretty well.
Think of Pokémon with a dystopian taste of a post apocalyptic look, something from Fallout, Mad Max, or other similar setting where the forum buttons, graphics, and icons look beat-up and burnt over and the colors are generally desert-themed. I used a few pictures from post apocalyptic pictures along with movie picture captures from the movie The Book of Eli, and it turned out pretty well and gave the whole forum a nice and gritty Fallout-inspired feel to it.
Eon Twins A.D. 2300
This one was a futuristic theme featuring Latios and Latias. Buttons and graphics looked more high-tech and generally tried to make the skin seem more like it was from the future.
Pokémon Urban Street
This skin had more of an urban feel to it with city and subway street themes along with using graffiti style text. Meanwhile, a lot of tarmac and concrete colored grays were used in terms of the color themes. Unsurprisingly, this turned out to be pretty popular with the community and I used it myself for quite some time back when I used to be there. I really liked the way this one turned out.
And there's some other ideas I have that I didn't actually put into action in the form of a skin on Pokémon Elite 2000, but Serebii could still easily use.
A Horror Skin
Something creepy, gothic, and unsettling would be pretty cool to see. And not something cartoonish as in Saturday morning Halloween cartoonish, but something genuinely disturbing and truly horror themed, as if Pokémon and the Silent Hill franchise suddenly had a crossover (which would be awesome, but will probably never happen).
A Military Skin
Essentially as if Pokémon and the army had been combined. Stencil lettering, gritty textures, camouflage background splashes, that kind of theme. Would probably be a hit to the Call of Duty and Battlefield fans that also frequently visit this forum.
A Fantasy Skin
Something that makes Pokémon look more like it came out of Lord of the Rings, Game of Thrones, or that kind of thing, using some of the more Celtic style fonts out there with fantasy settings and themes.
These I think would be pretty cool and pretty flexible in terms of how they could be planned out and constructed. I'd love to help as well if you'd like. I've done skins before and I have loads of graphic design experience.