This suggestion will effectively function as the highest tier “Guild Mission” with similar rewards and some extras for winning. At the Lions Arch WvW Asura gates, a new gate will be added for a Guild Mist War that is only accessible to Guild Officers.
If the Officer of a guild with sufficient influence points enters the gate, they will be automatically added to a que of Guilds seeking to wage a Guild War. From this menu, they be able to issue challenges to other listed guilds. Once a challenge is issued, guild officers from the challenged guild may choose to accept or deny.
Once accepted, everyone in the guild will get a 10 min warning notice that a Guild War is about to start. This’ll give em time to vendor and get to lions arch. At the end of the 10 minute warning, the Guild War Asura gate will become accessible to all the members of the guild.
On the other side of the Asura Gate will be an open world PvP map. It could be a carbon copy of Eternal Battlefield (but you would need 3 Guilds for the war then) or a special map specifically for the Guild War (Which would be better). Complete with 1 Keep per guild, a Stonemist style cap point in the middle, towers and supply camps.
Some key differences though will be the time and cost needed to upgrade positions will be greatly reduced, and I would suggest that Yaks either move faster or spawn at a greater rate.
This is because unlike a normal WvW match that runs for a week, a Guild War will have to run for much less. I would suggest 4 hours, but you could conceivably shorten it to 2 hours, or if you want to have fun with it, have the guild agree on a time limit based upon a scale.
The Guild that loses the guild war will just get guild achievement tokens. The winning team will get the bonus chest and tokens, a Grand Chest accessible in their Keep, and bonuses for WvW (like being able to carry 15 supply and so on) for a day.