Not sure how hard this would be to implement, but I think a two tier assignment system would work.
1. Divide everyone into sections based on GP similar to how guilds are separated in TW.
2. Assign opponents based on the top 60 toons (6 teams on defense, 6 teams on offense worth) using the number of geared and zeta'd toons on their roster.
Example: I'm at 3.5 million GP. I have 31 G12 toons and 36 G11 toons and 33 zetas. How hard would it be to come up with an algorithm that would assign me an opponent who has a 3.5 million to 3.75 million GP with between 30-35 G12 toons, 25-35 G11 toons and 30-35 zetas?
For the record, my opponent had a 3.5 million GP, 33 G12 toons, 22 G11 toons and 30 zetas so it was a pretty even match. I was unable to clear their board because I misjudged their Bastila team's speed that they had set on defense. Even if I had beaten that Bastila team I would have lost the match. My total max score would have been 1,292 to their final score of 1,293. I went weak on defense so they swept my board on single battles. They did the same except for their arena Bastila team which proved to be my undoing.
Even matches are fun if done properly because it will come down to making sure you can not only beat the opponent's teams but that most of your toons will walk away alive and with protection if possible.