I had a little bit different idea last night so I'll throw it out there.
Break up the arenas like this:
Early War Axis vs Allies
Mid War Axis vs Allies
Late War Axis vs Allies
Run a series of maps through these arenas that represent each of the campaign fronts with all of the respective era and country planes available to each side but limited to the countries involved in a specific front (historical match-ups. i.e. one map might be eastern front, another western front, another Pacific front, etc.). Make each country's capital city uncapturable but the rest open to capture. When one side is beat back to their capital city the map re-sets. These arenas would only comprise two "countries" and specific planes would only be available at specific bases.
Training arena
Liesure arena
Main arena
All new players would be required to enter the training arena and are only allowed to leave once they can take off, fly a prescribed course, and land safely. Maybe add a firing requirement at a stationary target. This at least gets the basics out of the way. Otherwise the TA would be as it is today.
The liesure arena would be the next requirement. In the liesure arena there would be no black-outs, red-outs, no base capturing thus no re-sets, and possible other "easy-mode" enhancements. To be able to graduate to the main arena or to one of the Axis vs Allied arenas a player would be required to reach a pre-defined goal (total kills, kills per death, Liesure arena ranking, etc.). In addition, the plane-set might be limited.
The Main arena would be what we have today but once there there would be no stepping back to Liesure.
While EW, MW and AvA are mostly dead arenas at this point, I think combining them in this way, and combining their scoring and play styles with the MA would give them a boost. I know I'd give them a whirl.
In addition, I think the two-step graduation process to the MA/AvA's would appeal to the gamers familiar with "levels" of play, better prepare them for the realities of the MA and decrease frustration over the learning curve.
Just an idea. Flame away.