Ok, so ever since the arena splits, EW, MW and WWI have been ghost towns, which is a shame because it's great having arenas where early machines can be competitive (IE, the F4F-4 is pretty outmatched in LW and to a lesser extent MW, but is quite competitive in EW).
While I don't expect the other arenas to EVER reach the population levels of the LWMA (which I think is less to do with everyone wanting to only fly late-war monsters since there's quite a few popular MW machines than it is a combination of 1) everyone is playing there anyway, and 2) being the closest to the "classic" main arena from before the split) getting a healthy population in those two arenas is going to be good for the game.
The question is, what sort of adjustments can be made to draw players in?
One for certain would be to have new maps with bases closer than 25 miles in EW and MW. The aircraft are slow enough that even at those distances it takes a long time to get into a fight. Closer bases will mean quicker action, making it easier to find a fight.
As for WWI, been a while since I've bothered logging in there, but from my experience its main problem is the plane set. You may as well rename WWI the Dr.I arena, because that's practically ALL you see. Perk the Dr.I and start flesh out the plane set (I mean seriously, why is there no SPAD or Nieuport, or an Albatros or earlier Fokker biplane than the D.VII?). And frankly, WWI would benefit IMMENSELY by having more gameplay options than just furballing. Put up some balloons or Zeppelins to attack and defend.