Rikifive

The Magic of Friendship in her is so strong, that it explodes! Literally!

Thanks! 

This UI looks like it works really well!  It doesn't feel like they're mana bars so much as ammunition for different weapons, but it works.

So...Twilight's magic missiles have unlimited ammo but short range, Pinkie's streamers are rapid-fire but individually don't do much damage, while AJ's fireballs have low ammo but deal massive damage?  Interesting balance; I can see different abilities working well against different kinds of opponents.  Bring spread-shots and the ability to cancel enemy projectiles into it, and there's quite a range of possibilities for how to balance it all!

Question, though:  how do the mana bars/ammo bars get replenished?  Do they recharge over time, or does Twilight need to grab items in order to recover ammo?

It works, it works ~ after I tested it all in game I can totally say it's way better now. 

Yep, that magic missile acts as a default spell. Has infinite energy, but is the least effective spell.
Pinkie's streamers are fast and have 'auto-fire', so you can just hold the button down.
AJ's is yeah, self-explanatory. Deals high Area of Effect damage, but way less maximum energy.
But that's not all...

Apart from the 'default' magic projectile, there will be 6 additional spells unlocked through the story, each representing the element of harmony. 2 of these are already revealed, the rest I'll keep for myself for now, as I don't want to spoil that much right away.

As you said, each spell will have its own usage, performing better in certain circumstances. That -- and of course just having fun with these.  

On top of that, each spell can be improved along with progress through leveling up. Will it be maximum energy, range, speed, amount and size of explosions or simply damage. Along with stats, spells may also change their appearance and behavior upon improving, especially Element of Loyalty. 

The energy is replenished by picking Elements of Harmony, that enemies drop, as seen here:

(note: this is a gif from previous updates) 

Each Element of Harmony replenishes some energy of its corresponding spell.