I think there is more interest in WWI flying than current arena numbers are reflecting.
First, another plane or two added would definitely get people back in. A Spad is the logical choice I think.

Second, add some aerodromes for the WWI arena situated on 5,000 foot hills. Let me explain-WWI planes, not being powerhouses of performance, really benefit from having some air under them to cash in for energy in maneuvers.
Without alt, they are often stuck at just doing sustained flat turns at tree top level. But it takes almost five minutes just to climb to 5000 feet in these little planes. So add give players a 5K jumping off point, that will give furballers several minutes to spiral down in a swirling fight before ending up on the deck.
BTW, it should be pointed out that the guy who just took off from 5K wouldn't have some sort of enormous advantage over the guys down on the deck. WWI aircraft can't directly dive that far and come in with a huge energy advantage like a WWII aircraft can.
Of course, there would still need to be aerodromes on the deck for RTBers.
Third, how about giving the WWI arena some kind of objectives? Not the base-taking bit, I don't think anyone wants that. How about some zeppelins floating around, maybe driven like a CV, for each team? Something to attack and protect.