I've also been assuming that something like this would be a logical next step. Like each regional chart and the worldwide chart would all have prize tiers just like the contests. On top of that I'd think that they could expand mojo to be spread throughout more aspects of the game, as in different amounts of mojo are rewarded in the tiered contest prizes along with adding small amounts wherever else as well. (One small idea: Maybe pair up xp and mojo so that whenever any amount of xp is gained you automatically also gain 10% of that amount as mojo?) And then whatever the schedule would be for awarding tower prizes and resetting the tower's charts (I was thinking more like once a month, but once every 2 or 3 months would make sense too).
Edit: Random idea that came to me just now: Maybe just for the vanity / for the "bragging rights", they could add some sort of badges when viewing player profiles in the tower which shows off their personal best reward tier ever gained from regional tower leaderboard, from worldwide tower leaderboard, and from each individual contest leaderboard. Plus also some badges representing total long-term performance in some way... Like a top tier result for a tower ranking or for a contest would be worth 5 stars, 2nd tier is 4 stars, 3rd tier is 3 stars, 4th tier is 2 stars, and last tier is 1 star. Each performance badge shows the total number of stars earned for that category. A new tab in the tower could let you view all players sorted by star counts, with a filter for selecting to view all players or just a certain region, and a filter for viewing players ranked by all total stars or by just a specific category's stars. Maybe also have badges which show just the stars earned on the three most recent tiers completed for each category, and the new listing has a priority mode to bump players' positions in that listing based on the average of that recent activity. None of the badges or the new rankings tab would ever be reset or used for rewarding prizes. It would all just be for show.