Other idea: Start looking in the boys' section first, as I agree the teen/preteen boy clothing sections are usually the most limited. Then once you've picked a theme, look for adult PJs in the same theme for you and DH.
So for example:
H&M have the starting point for a New york theme, Harry Potter theme, stars theme, technology theme (wifi/emoji) Marvel, Ghostbusters, NASA, Pac-man.
www2.hm.com/de_de/kinder/produkte/jungen-134-170/nachtwaesche.html
At Next you've got:
Orange, Mountains, Wi-fi again, 2 different coloured camo print (could then get green/brown or green/pink for adults)
Sainsbury's: Marvel, Pokemon
Adsa/George:
Sonic the hedgehog, Harry Potter, Stars, sharks, bears, mario kart, space.
Gap:
Dinosaurs, sharks, emoji
John Lewis (excluding single sets over £20)
Dragons, Sharks, space, dionosaurs, City scene, tigers
Debenhams:
Bears, mammoth, wi-fi, space.