1. Larger scale arena battles.
When I first played GW, there were a few maps in the tombs (now HA) where you had 6 teams going at it. There were large chaotic battles and that was great (even if I didn't do all that well on my few runs in there). Anyways, if possible, bring it back. In fact make it its own arena type. You can either go into HA and have mostly one on one with a few larger battles or you could go into a larger arena set up.
You could break this down further into 3 ideas. 6 way free for all (of various types) is one idea. Basically 6 teams enter, one team leaves. Then you could have a 3 way cooperative map or two. Three teams each made up of a pair of groups. Then you could have a grand melee. Two sides of 3 groups (this would be great for factions), or a supersized version of the faction battles.
This could possible give prizes for any team that wins 4-7 rounds like the HoH.
2. More options for team arena and random arenas.
Include some 3 team, 4 team, and 6 team maps as well. Mix it up a little.
3. The pit.
This one is simple. Put a pair of teams in a small arena with few features. A small circular room would do. Let them start off in each other's face with little place to go.
4. GvGvG[vG(vGvG)]
Ask yourself a simple question, why does Guild versus Guild have to be limited to two teams at once? If you've got some 8team arena maps, you can put them to perfectly good use as neutral ground for multiple guilds going at it at once. At times when it is harder to find an even pairing for two, you can make it more than just one on one. It might even provide more of a challenge for the upper-tier teams if they suddenly have to deal with more than one team at a time. Plus who wouldn't mind seeing more than two of the best teams beating the snot out of each other at a time.

5. More hostile arenas.
Okay, we've got arenas with icy areas and areas that set you on fire. How about more?
A maelstrom arena where random areas periodically get hit by a skill level 0 or one maelstrom is a possibility. A chaos storm arena would be a nice variation on that. Like air? A gale arena could work: everyone has a 50% chance of being knocked down every now and then. Maybe a thunderstorm arena with falling rain and the players (and adjacent) have a slight chance of being hit by lightning. Some arenas could force you to fight with half energy/ade regen or there could be an arena with no healing at all.
6. Helpful arenas.
The arenas don't just have to provide a drawback for each team. Maybe one could give everyone in it a speed boost. Or mending/healing breeze (or something like that). Some arenas could come with nature spirits. Maybe there is a dwyana's arena where all healing is doubled and necro spells are at half skill level.
7. Treasure hunt.
This would be like capture the flag/artifact, but you wouldn't know what it is exactly you are looking for in the beginning. There could be several items in the arena to collect and bring back but your team only needs to find one (and it could be different from the other team). Most like objects found win.
8. Scattered teams.
Why do teams always have to start together? Why not let everyone start in an individual cell and make them work to meet up with the rest of the team.