The System:
1. INITIATION. You and your party kill a random monster boss (Charr for example), and OUT OF NOWHERE...a guild flag pops out from the sky!, along with a window saying: "YOU HAVE KILLED X! HIS GUILD, CHARMANDERS OF ASCALON, CHALLENGE YOU! (300 enemies)"
2. DRAFT. Next comes the recruiting part: your guild, which might not have enough members for this huge fight, will post up a message just like the observer mode system in which other guilds can participate in your "guild" fight. Random guilds join, and you guys begin the battle.
EDITED SUGGESTIONS:
(2.a) ORGANIZATION. A pre-battle period would take place in which the humans can form teams of 8. This way, the monk job of healing would not be so stressful. Maybe we could toggle between healthbars of different parties, so if one party is dying, a monk could just click on their party leader to see their health window.
3. BACKGROUND. Monster guilds would be lead by respective leaders in the pve story, to give the plot more practicality i.e. Bonfaaz Burntfur would be the level 30 guild leader in the center of the battlefield. Drakes would receive glint, Titans the Lich, Stone Summit the guy who killed Rurik, and so on and so forth. Human guild leaders would be raised by 5 levels and given some kind of status to make them look godly. (and charr would be raised to level 20)
4. BATTLE. Charr would battle in a HUGE field, with respective environments and strategic points. (bridges, towers, catas) all that good stuff, and a fortress in the back where guards lie in wait. You and your fellow guilds must obliterate the monsters (which can be controlled by devs) to reach step 5....
TOGGLED MODES:
(4.a) FLANKS (STRATEGIC). The ability to gather a team and start completely somewhere else. Instead of joining the big battle, jump in the portal "West Battlefield" and take your position. Form a party of strong soloers (Racthoh ahem ahem) and set them loose in the monster "Guild Base", taking on the strong strong strong guards!
(4.b) DISTRIBUTION MODE (credits to Banebow). Guilds start off in 6 spawns resembling a circle, in which monster patrols surround each. A main force is in the center, and guilds can go about different ways in killing the force. ("Do we kill the patrols and team up? Maybe we should kill all at the same time for faster rewards!")
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5. REWARDS! Very rare items, maybe new "red" items like "The Furburner of Bonfaaz"..I dunno! It could get creative!

Please critique! I dont care if you totally hate this idea, just tell me! And give some tips to why its so goshdarn bad. Thanks!