If you make it free unlimited play for a Panzer....are you ready for 1000 Tank players? Isn't the Arena limit 500 players?
If you want to get the numbers up, How about everyone go out and recruit one new player? Back in the AW days, a co-worker introduced me to the game and I've been hooked for 20 years.
I personally feel that it's the hardware cost side of the game that limits it's access, not the monthly fee. You need a descent computer, and at least some dedicated peripherals, which many coming to investigate won't have. The game doesn't play well if you are new and trying to use a keyboard. So you have to buy enough equipment to make it playable...BEFORE you play it. Very few are going to go out buy a twisty stick with hat switches just so they can try AH3.
You can't take a guy to the top of a ski lift and tell him to slide his way down the mountain on the soles of his tennis shoes and think he's going to get hooked on Skiing.
We...as a community....need to reach out and recruit people, sit them in your chair get the headset and your TrackIR on them let them play around and see how cool it is. Then fly and let them watch a quality dog fight, or other in game action. Get 'em hooked and sign 'em up.
Recruit people that have a descent PC, and a love of WWII, or planes...or even a love of gaming.
Good luck!!