Have often asked for the Studebaker truck and trailer to be implemented some day.
Basically the truck carries ammo for what ever trailer option is chosen in the hanger.
Katyusha trailer (land mode only with min as well as max ranges) (30 sec reload per 16 rocket salvo)
88mm field gun HE, AP (sight only) (as per tiger)
5" field gun HE (land mode and sight) (as per ship)
You could also do other stuff with it but thats an easy minimum starter.
You tow the trailer to a location and deploy it (which may take some 30 secs) but puts you under camo netting with no icon.
If it was lightly perked then a formation option might make it interesting
edit
In fact we already have 5" spinner rockets modelled on the PT boat. I am not sure if they are the 5000 yard or 10000 yard range versions. The ECO 80 carried 16 of them. So sound files and flight dynamics are already modelled.