I mean 'beating a world boss spawns a Loot Caravan' is one of the most 'obvious' designs for this, right?
The biggest issue with this is the zerging potential. I guess you could have a limit on how many people one side can have, but then you'd have to tie the caravan to the pve raid group because otherwise the attacking side could register some of their people for defense too and fail it on purpose.
And in the usual context of "pvers don't pvp", that raid group would be shit against the same-sized pvp-centric group. Maybe you could assign someone else for the defense and you'd point to your guild's pvp raid group, and maybe they'd be teleported to the picked location of the caravan (same as the newly updated caravan spawn system), but at that point we're so deep into maybes and unknown sides of the mechanics' design that I dunno if it would feel good to play it out.
You'd also have to make those caravans sink-less, or at least I feel like people would hate it if their well-earned loot poofed out of existence after a super difficult boss fight that took a ton of attempts.
Oh, and there's also the potential of the defending side registering their own players (or mercs) for the attack side and filling out the limit so that their enemies couldn't do the same.
And I feel like all of these issues would be non-existent if the game supported some proper agro control of mobs and/or a few different pvp mechanics before the fight for the right to farm the thing (or at least the dibs to farm first).