/facepalm
This isn't about how its unbalanced, there's already a thread on that, this is for a possible (yet more difficult) idea I have. It includes mission objective change, along with a map change. Prepare for a good amount of text.
Have both sides have a fort and name it, the forts of aspenwood. Small name change just to have it fit. I drew a crude sketch of what the map would look like. The Lux fort will look exactly the same, but I grew lazy....think symmetrically.

Before I get too much into the layout let me lay out the major goal of the arena. Be the first to collect 10 (number can vary) war plans from the enemy fort. Yes this is a infiltrate and escape mission. With no Pure Defend the fort, both sides have an active goal and aren't forced to play pure offensive or defensive roles. After 28 minutes, the team with the most plans wins. If equal, then the next team to get one wins.
War plans are bundle items, and when someone carrying one is killed or drops it, The bundle item stays where it is dropped (unlike amber in which it disappears after a short time). Until a member of a team that owns the battle plan picks it up of course, then it goes to where it was in their fort.
Optional part
While holding a battle plan you are affected by " Get this to the Master!" where snares cannot exceed 66%, and you are buffed with a 10% speed boost. However while under this affect you cannot block attacks due to recklessness. Either that or remove the affect all together. I think it would be fun.
End optional part
Now as for where the plans are located, they are random and you must find them. Sometimes they are carried by NPC's, sometimes they are laying around the fort, but they are ALWAYS in the fort. If an NPC is somehow dragged out of their fortress, if they had a plan, it will transfer to another NPC.
This part is optional.
The last Battle map that you can collect is collected from the enemies Master, located in the rear of their base. Once the master reaches a health of 5%, he drops the map and turns untargetable. If the battle plan is recovered he gains full health/energy and is cured of all status affects. The master is surrounded by 1 ritualist and one monk, and he himself is an elementalist/warrior with earth magic and defensive stances, and watch yourself.
End optional part
The NPCs in the map are balanced teams, spread out. Including one monk, a necro, elementalist, warrior, ranger, and assassin or ritualist. There are a total of five groups, and each group will respawn after a certain amount of time. These groups are marked by orange blocks. Apart from these groups there are solo npc's such as rangers at the fort's walls. These have a very very low chance of dropping a battle plan and do not respawn once killed.
The masters are located in the back marked by black squares.
Instead of two Doorways, there are now three. The middle Is Large enough for rangers or other ranged classes to be able to target almost anything in the area, but its open enough where AoE spells wouldn't be such a problem. The two outer doorways are so thin that they are subject to AoE spells and traps, but is almost impossible to hit anything with a bow/spear from above. However since there are 3 passages you must be wary of how you spread your team. 2-4-2 or all rush through center? There are no physical doorways to block passages.
There are two portals (marked by blue circles), both leading out of the fort and to the sides slightly (more exaggerated in the picture.) In the center there is an amber mine that is in a ditch, so you can only enter from the southwest or northeast, or shadowstepping. For every 8 amber you collect, you increase the chance to get a battle plan dropped. For every 4 that you get, one of 4 groups spawn near the gates that defend them. In this format the npcs do not move, so no worrying about them getting stuck in a path from an exploit or such. There is no capture the amber mine, anyone can use it.
Luxon center gate. Two siege turtles with 3 warriors
Luxon side gates. One siege turtle with 4 warriors
Kurzick center gate. Two Juggernaughts with 3 rangers that have make haste.
Kurzick side gate. One juggernaught with 4 rangers that have make haste.
The siege turtles keep siege attack, that cannot attack foes near it, and loose the enchantment affect and carrier defense (Since they do not move, no need to harm the physicals)
The juggas also loose carrier defense (Dont think they had it FA though anyways) and gain an PBAoE skill that matches siege turtles attack. The turtle has warriors to make up for its lack of Melee, and juggas have rangers to make up for their lack of range. Make haste is only cast on juggas and not humans.
That's my rough idea for a new, balanced FA....though Im not sure how balanced the jugga vs turtle will be... That may have to be adjusted.
Flame away or help make it better or discard it. Your choice.