Stitch
The guild hopping could be limited by putting a time limit on when you can join another guild after leaving one, say 5 - 7 days.
So, if some guild is planning an attack, they can invite their friends or even hire someone to join them a week or two before the attack itself. And thus gain some significant advantage over defending guild. With no way for defenders to counteract this.
This idea being implemented would left smaller guilds at mercy of the larger ones. If your guild has less than 15-20 members with mastered combat skills, you have no choice but either disband and join larger guild or quit PvP at all. Not mention begging for cats from large guilds and killing faction politics and interaction. Factions would lose any meaning and become completely RP things. Every guild would be for itself, if not consider invitation exploit. And what about newcomers that would like to create their own guild? They would be enforced to join exsisting guilds instead to get access to the cats.
IMO, GvG option shoud be implemented only in form of official agreement between two guilds that willing to go for GvG.