I've posted this before but I think a tiered system would work. Give everyone WWI for free to start to learn a few of the basics. Add EW and the TA for $5/mo., add MW, AvA and DA for $10/mo. and add LW, scenarios/events and access to perk rides (not available with lower subscriptions) for the full subscription price. This is just an example. Access, money and time can be tweaked.
This get's people gradually warmed up to the game, fills some of the emptier arenas and might encourage more high ENY plane useage in the LWA since many will cut their teeth on EW/MW planes.