I haven't thought about how to optimally set levels, or about e.g. xp as sublevels. I have thought some about the "merit badge" approach. Subcategories of Anna's 9 branches? And from my notes "Should badges have levels, like level 1: you can talk a good game, level 2: you've beaten others in contests, level 3: you are good enough to be a target for those trying to beat the experts, level 4: you are the current champion amongst level 3s." This makes sense for some things but not others - the ability to do 5 pushups for instance is good, but being able to do more pushups than everyone else may not be.

I think I would be less interested in obtaining some super-broad "Level 4!" with checkpoints than in embarking on a 3-month journey to achieve my "You have at some point demonstrated at least apprentice-level skill in novel estimation problems!" badge.