WithADanceNumberPL + KOTH. Alright so the cart works like a king of the hill point. Then it just moves by itself around this weird ass track and so the fighting dynamic of the koth point is changing constantly. Cart gives minor heals to team who owns the point (no ammo though). Team who doesn't have the point gets a faster respawn.
Just tried it in Hammer, this is definely possible. Only problem might be that the cart dispenser gives ammo, and you can't disable that, at least not without sourcemod plugins.
Now if only I was good at making maps...
EDIT
Just because I suck at it doesn't mean I can't give it a try right http://i.imgur.com/YDufLsu.jpg
EDIT 2
Update, already tested the map 1v1 while messing around, trying to have a bigger test later today http://cloud-2.steampowered.com/ugc/579001704000405771/DCFFA2DD9C95D27008206EA9CA4DC4E690F3DA85/