OK So here's another suggestion for a slightly mysterious MegaMilestone - or possibly two.
How about this? For every MM that you manage to get Ranked in the Top 100 for, you get a new Mega Milestone which for the purposes of this I am going to suggest you call a Brighty. You could take this further and give MegaBrightys for each one for which you rank in the top 10.
Fairly simple and could be nicely mysterious to anyone who hasn't read this, and more so because I can't think of a good reason not to give a bonus one if you get into the top 10 for MegaBrightys or top 100 for Brightys.
Owing to the numbers involved I would show them as soon as one was earned.
If you like the idea but hate the name then let me know, I could easily suggest another.
Just a thought.