Due to the fact that the ranks purchased are lifetime ranks, I think allowing players to purchase temporary ranks with their in game points would be a wise economical choice. Say spending 1,000 points gives you diamond for a day, or next 10 games (I don't have specifics, just the general idea). They could receive cubelets and an XP multiplier for the duration of their rank, and get to vote for different maps and item types, as well as having a temporary rank in chat. Of course the friends and weekly bonuses wouldn't be available. This would give players a glimpse into what owning a rank would be like, and I believe it'd be enough incentive to persuade more than a few players into purchasing lifetime ranks.