Having a semi-structured form of this would also be really good imo. Starcraft 2 sorta has this -- there's a UI section similar to ladder that pits you against increasingly harder AI's, but it's just a single track and doesn't go far enough imo.

I can easily imagine a set of 'tiers' of challenges, where the bottom tier is super easy stuff for beginners and then each tier up increases the challenge until you're fighting a bunch of hard-level AI's simultaneously or whatever. Or maybe a Mario Bros-style world map that has various different challenges in different directions.