A system where the guilds can recruit some mercenary to help them in the guild wars days. For example: a guild with 70 members, have only 60 members online in the GvG day. But, with the mercenary system, they can recruit 7 members (10% of the max of the guild). In this example, the guild can play with 67 players and can be more effective!
A lot of people want to do PvP, but some not every week and a lot of guild want to play with more players.