I'm sort of new to this game, but experienced the Christmas event. I would suggest more similar and not just American Holidays, but world wide Holidays, even events out of this world like the movie Outlander, make up fictional events, themes, instances,etc...
Create suits specialized to the themes, make scenerios, and or instances to win gear and weapons or special buffs. The Christmas theme had a lottery wheel that you could pay gold for more spins and chances to win other items, if the items are worthy and players can win a reasonable bit of chances they will spend real money. If the odds are not in thier favor, don't expect repeat money spenders.