I do like the idea, and you're right it's something that people don't seem to have explored.... Even more surprising, given the semi-recent Wonder Woman film
Foundry do British & German Trench raiders
Renegade did (as they are currently OOP?) early war trench raiders for British & German stormtroopers
Brigade games do British trench raiders (& American?)
Old Glory do British, German, American & French trench raiders + Characters
Scarab miniatures did (as site currently down) Austro-Hungarian trench raiders, and a solitary 'Werid WW1' zombie robot type chappie
Some of the Footsore & Artizan VBCW brigands might work as suitably ragged raiders depending on what your plans are.
Empress stock the Tsuba miniatures German Revolution line which might be handy for characters, levy (some of the civilian uniformed figures?) etc
Priests/cultists are fairly ageless
Pulp figures do 'pulpy' German Zepplintroopen and stormtrooper types
Ironclad miniatures do armoured VSF British Naval landing parties and Electro cannon.
A lot of the West wind SOTR werewolves etc would probably work with very little modification
I guess it all depends on how 'weird' you want to go and what flavours you want - mad science, occult strangeness, alternative history, etc etc