As much money as I've blown on this game, I would personally would love a subscription to it. I know there's the factor of "Free to Play!" But at the rate that I play, F2P doesn't cut the butter for me. All that I propose is this:

8 - 12 US Dollars per month:

3300 Crystal Energy.
Zero cost for elevators.
1 additional weapon/trinket slot unlocked, allowing the subscriber to equip 1 trinkets and 3 weapons.