A campaign focusing on one-off field conversions, oddball conversions, specialty vehicles etc. Such as Rommels Funnies, US Army Funnies, maybe even Hobarts Funnies, some examples:
Bren Carrier with 37mm Pak
Matilda with Pak
Sdkfz 251 with Calliope rocket launcher ( US Army modded a knocked out Sdkfz with a Calliope mounted on a Pak 40 to use its elevating mechanism)
Pz I with Stuk 40
M3 Stuart with Flavierling
Soviet Tractor Tanks
And so on. Given the German propensity for repurposing captured vehicles during WWII there may be a slight bias here but hopefully people will dig up some new oddballs.
The Rules and Regs
Can be any air, sea or land, can be a fully released kit or a scratch built conversion, any era, there would just have to be photographic evidence of its existence. Cannot be a purpose designed vehicle, has to be an existing vehicle that has been modified.