I also considered a counter with "Counter target spell if its controller has fewer experience counters than you," but that's just a counterspell if they're not using experience counters, and I'm not sure the gameplay is good even if they are.

See Lesson in Pyromancy and Lesson in Recruitment for other cards with a similar mechanic.

All of these are inspired by Kalemne.

Huh. Technically, these cards could all share a type, then use the amount of cards with that type in your graveyard. But this works well, too. It certainly gets around self-mill...

Alternatively, you could make something like this:

Lesson in Pyromancy
­
Instant
Master (Exile this spell as it resolves.)
Deal 1 damage to target creature or player for each spell you Mastered.

Admittedly, it wouldn't work well with creatures (except for leave the battlefield effects, or an alternative to sacrifice.) But maybe that's not a bad thing. Too many cards that allow you to Master might get out of control.

These were designed specifically as a riff on the new Commander spoiler.

I do like your idea, though, for the record.

The whole mechanic is wonky - even with just the commanders -, because the more cards you create that gain you experience counters, the more powerful it becomes.

It's like energy, but without ever spending your resource.