Tumbleweed
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.
Yes it would leave smaller guilds at the mercy of larger guilds, which is the way it should be. If a guild can't hold an OP by itself, then it doesn't deserve it. Your notion that a small guild has to quit because of that though is just silly. TI grew from 2 members with no OPs and no support from any other guilds to where it is now and any other small guild can do the same if they put some effort into it. If they don't want to put in that effort why should they be rewarded with OP's? Factions are already meaningless in OP wars as it is since anyone can fight for any side they want to. Just because most battles are fought along faction lines doesn't mean they have to be or even should be. As for new guilds see above about TI starting from nothing.
The only real issue would be the guild hopping. That would need to be taken care of first. Although if a whole bunch of people all of a sudden leave their current guild and join a different one, it's a pretty safe bet that something is about to go down.