Just had a potentially cool idea.
What if they could allow you to set up a sort of "mercenary guild" for PvP players?
You'd be able to purchase a stall based upon the AP your guild members earn (the game would track total AP earnings guild-wide, which would be "spent" on bids, but only be spent if you win the bid), which would then appear in the lowest population or lowest scoring faction's home area for a week. (Similar to the guild merchant stalls)
The stalls would have an option for another guild in a different faction to place a "bid" on hiring your guild's services for the duration of your guild's stall purchase. They would only be competing with their faction and the opposing one (or, you could restrict it to the lowest pop faction, or the lowest scored faction, in a given week), and once they buy your guild's services, you are flagged as an "AD/DC/EP Mercenary" for the duration of the week, and your members would spawn in the purchasing faction's zone to fight against the other two factions within the given campaign.
Whether the buyers must use gold or AP or something else, and how much the Mercenaries get for their services, I don't know. I think you'd need to cap the number of hired guilds for any faction to a small number, maybe one or two, per week lest you throw the balance too far off. I also think the hired guild would be restricted from any emperorship titles when hired, but would maybe gain PvP gear bags (the ones usually purchased with AP) as rewards for exceptional performance. If the idea stuck, there could even be a separate mercenary leaderboard that tracks AP gains during "hired periods", with special rewards at the end of a campaign.
This would give some of the larger guilds on high population factions a chance to keep earning AP, fight new players, and would also bolster the lines for the low population factions. It could lead to interesting guild dynamics within the current PvP system.
I am of course just brainstorming here, so feel free to agree or disagree, or to expand upon this idea. I know there could be potential for trolling or abuse, but it would iron itself out as guilds make a name for themselves as mercenaries. The ones who troll wouldn't be hired, and the ones that perform would become in-demand to assist lower population factions. If based on scoring, it could turn the tides of a campaign in the direction of any faction who hires the right guild.
If you have any other ideas to help with campaign balancing, also feel free to post them here.
M'iaq the Honest- PC/NA
EP Khajiit Nightblade
Guild of Shadows