I think another good theme could be "cartoon". If you look at

"Worms Armageddon"

"Earthworm Jim"

(even the cow doesn't look misplaced)

or "World of goo"

you may get an idea what i'm thinking of. Inside this cartoon World you could easily mix and include some of the other themes. Ancient warriors can fight dragons in space, Dogs can beat up evil postman while protecting the world from evil platypuses...

The possibillities are endless ;)

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For legal reasons: The pictures are not mine. All rights belong to the original creators. I found them only with the Google image search and linked them as examples