I may never run it, but lets take elements of Madoka Magica, and mix them in with Perplexicon!
Keep your heads on, folks: THIS CAN'T POSSIBLY END WELL POORLY!
Rather than Pool, you wind up with Potential. Potential determines how many points of Pool you have, but it never regenerates. If this hits 0, you die. It also goes down by 1 every turn you're in combat at all.
Should you die by loss of pool, you'll become a Familiar. You've got a 1/10 chance of holding a Grief Seed, you create a gateway into a world under your dominion, and you gain limited but powerful magic based around the world you're in. After spending enough time in your Labyrinth, you'll become a full-fledged Witch. Witches are able to bend the rules in their realms (such as making it harder to cast magic) but only in a way that affects everyone (making Fire magic triple in effectiveness may be useful, if say, you're impervious to Fire and can use fire spells). They can suppress others with their will, hide (someone in their realm must roll to seek them), and take on inhuman forms.
Dying by other means leaves you out of the loop, but you go to a different counter: Walpurgisnacht will come whenever the non-witch death toll reaches a certain, hidden number. Walpurgisnacht is well worth slaying, however, as it drops Grief Seeds for every player that went to it's counter.
Every player spawns with a single Grief Seed. They will also spawn in an area with a number of Witches and Familiars, n-1 Witches and (nx3)-1 Familiars where n = number of players. Players may fight each other, team up, or stick to slaying Witches solo if they like- Familiars have a very low chance of dropping Grief Seeds, Witches always do, a Grief Seed can replenish a mage with 2 points of Potential back to full from 0 before becoming useless (not sure about how many points it should be yet.)
Battling another mage may or may not be a good thing- waitlisters won't come on for three turns after a death caused by another mage and you can establish territory, take their Grief Seeds, but others will be less inclined to trust you and you're fighting someone likely equal to you. You can fight Familiars, who aren't as difficult as Witches, but rarely grant Grief Seeds. You could fight Witches, which is dangerous, but will keep you in the fight.
The generated Familiars have a random number of turns before becoming Witches.
A Witch will generate a Familiar every third, fourth, or fifth turn of existence.