Creating new spells

One of my players wants to make his own spells, so we're working on some moves for that. Here's what we've worked out so far:

When you cast a spell you have not mastered, discuss the particulars of the spell with the GM, decide what level the spell will be, and roll +Int. You may only attempt to master a spell of a level you can currently cast.

On a 10+ You cast it no problem, and hold +2 Mastery.

On a 7-9 you cast it but the GM gets to modify the spell in some way: Range, Effect, Targets, etc. The spell goes a little wild. In addition you hold +1 Mastery.

When you attempt to master a spell, take a few hours to commit it to your spell book, and spend your held +Mastery. When +Mastery spent is equal to twice the level of the spell, you may add it to your spell book. You may only hold Mastery towards spells equal to your Int modifier.

So if you have +2 int you may hold mastery towards two separate spells. If you'd like to start mastery on a 3rd spell, you must release the held mastery, erase the spell and replace it with the new one.

You may also gain +1 Mastery by:

Spending time in a place of power, studying your spell.
Asking beings of great power for understanding, at a cost.

Interesting concept.  I like your move, although it's not clear if they have to memorise the prototype spell when they Prepare Spells - I presume so?  Also, you'd probably need to rule that a new Cantrip requires 1 Mastery to perfect if you don't want an arbitrary number of zero-level spells being created.

After reading it, though, I started wondering how to do the same thing without a custom move.  You could simply say that creating a new spell could be one of the effects of a Ritual (with an appropriate cost or caveat).  Once created, if they memorise and cast it, you could require that they Defy Danger with Int (in addition to rolling to Cast a Spell), the danger being that their new spell doesn't function exactly as expected this time, backfires or otherwise behaves like a buggy 1.0 release of software.  You'd need to have something to decide when they can stop Defying Danger to cast it, though, like your Mastery value.