I actually like what White Knight Chronicles 2 did, though it was much more pay to win style, but you could "buy" buffs. You could buy higher drop rates, buy +exp, buy +gold, etc.
I'd certainly pay for shit like higher drop rates.
from wkc2:
Dahlia Ticket 2,99 Boosts by 20% the dahlia earned for finishing quests.
Experience Ticket 2,99 Boosts by 20% the experience earned for defeating enemies.
GR Ticket 2,99 Boosts by 20% the guild points (GP) earned for finishing quests.
Guilder Ticket 2,99 Boosts by 20% the guilder (money) earned for defeating enemies or finishing quests.
Item Ticket 3,99 Boosts the droprate and quantity of defeated enemies. See page for details.
Results Ticket 3,99 Boosts the number of items earned for finishing quests.