So, yeah, was having a think about this. I mean, the limited moves idea works really well for a game suitable for younger gamers, and the show's really very hot right now... so I started having a noodle, basing some basic moves around moves from other hacks (particularly Dungeon World and Monsterhearts), and things are starting to come together...

The basic stats are, Sweet, Brave, Sensible and Odd (though I may rename the last one).

Instead of getting Harmed you have Hurt Feelings, which are Loneliness, Fear, Anger and Pain. When you have a Hurt Feeling, you add a token to one of them - once you get 4 (in any combination), you are out of the current scene and mark one of the following Conditions - Isolated (-1 Sweet), Terrified (-1 Brave), Flustered (-1 Sensible) and Drained (-1 Odd). I'm going to work on some ways to heal people of Conditions, and one playbook will be the equivalent of the Angel in healing Hurt Feelings.

Speaking of which, I thought having the basic playbooks based around the Elements of Harmony would work really well, and then use the DW approach of having each playbook have a species-specific move. The Element of Magic, of course, can only have Unicorns. Each one then also starts off with a unique Move, and then you can choose others as you develop.

I'm also thinking of adding in a barter-like currency called Gifts, which can be used to get stuff done, and having the Element of Generosity be specialised in maing and using Gifts.

Anyway, here are the 4 basic Moves I thought would work.

Talk somepony around
When you try to talk someone into doing something for you, roll+Sweet
If they're an NPC, you have to promise them something first, such as a particular Gift:
10+   They do what you say; whether you keep your promise or not.
7-9   They're going to need some assurance that you'll keep your promise.
If they're a PC:
10+   Choose both
7-9   Choose 1
If they do it, they mark experience
If they refuse, they have to defy danger

Defy danger
When you stand up to a threat or hostile situation, roll+Brave
10+   Choose 3
7-9   Choose 1
You don't get hurt
Nopony else gets hurt
You don't get caught or trapped
The threat backs down and apologises

Show your knowledge
When you think you'd know something about a current situation, roll+Sensible
10+   The GM will tell you something interesting and useful about the situation
7-9   The GM will tell you something interesting about the situation, but you have to make it useful

Follow your heart
When you do listen to your instincts as to what to do, roll+Odd
10+   Choose 2
Your heart tells you a course of actions, if you follow it take +1 forward
Your realise something important, remove a hurt feeling you have
The course of action if clear and other ponies will understand it
7-9   Choose 1
The course of action will hurt one of your feelings
The course of action will hurt someone else's feelings
The course of action is strange and other ponies won't understand it

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I was also thinking of working on the MC section, and having fewer Moves for them to make, but making them more explicitally like the PC moves. I know this goes against the standard AW philosophy, but I thought a) this would make it easier for young (hypothetical) MCs to run the game, giving them a structure to work with too, and b) who cares? It's all about the hacking, and I think it would be interesting to try.

Firstly, cheers for the response!

Well, I'm bouncing around a few ideas (one of which was, in fact, an X-men inspired game, which I think is badass), but the main two going around my head are:

Wolfhearts: A game about young warriors and the tumultuous nature of young friendships (inspired by John Harper's the Wildlings)

and

Steelhearts: A game about samurai falling short of their ideals and the fallout that causes.

Particularly, with both these games, I'm really interested to see if I can get the AW/MH idea of limited or more specific moves to change the way the players view the narrarative in a different light. Wolfheart is about friendship and learning who you can trust and what happens when they trust is broken, while Steelhearts is about being in a vastly different sort of societal enviroment, and about emotional repression.

But damn, Mutanthearts, that's a fantastic name...

EDIT: Also, a Spy hack, I think. The dynamic shifts in power works perfectly for it, as well.