I've long debited, and I know Jorb and Loftar also have come up with several reasons to buff and make wagons slowly viable over the years.
There a few tech that I think the wagon should use, the inside of a wagon should act like a dungeon. When entering or leaving a dungeon within a claim you get the "visitor debuff", if the wagon was suddenly broken inside a claim the players within get "visitor debuff".
This would allow players within a "dungeon" wagon to be driven into and thought gates but still get visitor debuff as they leave the wagon.
This leaves how protected does a wagon door should be.
Mobile mode anyone can enter if they lockpick the door. As wagons can be handbashed during mobile mode, this would not be easily to abuse for combat.
The problem child mechanically is encamped mode. Siege machines (ram and catapult) can break an encamped wagon after an hour of drying off claim (not tested this since world 15 dryin times might have changed).
I'd rather a door that was unpickable while it's in encamped mode, but that would make it a pvp bunker, so maybe unable to enter it while in combat (what also removes cheeseable exploits with hunting), and takes 10 seconds to enter it while its encamped.
I think this covers most of the exploits I've thought of over the years with wagon interiors.