Google Doc with comments enabled.

You can just jump into the doc, or you can read here the elevator pitch.

Gea, the All-Mother, has shattered. Heroes arise to defend a Hometown from the Corrupion that surrounds it, and venture out into the wild to cleanse it and collect more of Gea's Shards. These allow the casting of magic.

Gea's Shards are d10 and come in four colors, each tied to an Aspect of Gea: the Red Shards of Gea’s Burning Heart, the Green Shards of Gea’s Nurturing Hand, the Blue Shards of Gea’s Warding Womb, the Yellow Shards of Gea’s All-Seeing Eye.

Using them will eventually turn them into Corrupted Shards: black d12's.

An average spell will require you to channel Gea's Mana. To do so, toss a handful of shards on the table: any that roll above your relevant stat (for example 5) channel one point of mana. Any 9 or above channel two points of that mana. Any rolls of 12 channel two mana of the kind you want, plus one of every other kind - as well as one point of Corruption, of which getting 7 is lethal (but there are ways to get rid of it). Finally, a roll of 0 will turn your colored Shard into a Corrupted Shard - more powerful, but more dangerous.

Here's the catch: if you don't have enough mana channelled, you can pick up all your failed dice and roll them again, along with any extra you may have kept back. Same rules, add mana - except that if you roll zero new successes, you lose control of the spell and it blows in your face.

I am open to all kind of feedback, but I am mostly concerned by two things:

Is this kind of free magic casting too free? Will it become unwieldy at the table, put undue stress on the GM or result in players always casting the same things defeating the purpose of an open magic system?

How exactly do I go about building a high-level game loop that keeps the players engaged? I have some ideas in the Doc, but that's all they are. I am open to suggestions. I would like to avoid building in extra mechanics to cover the non-magic stuff, or at least keep those to a minimum.

Credit: dice mechanic stolen inspired by this post by u/maibus93. His Google doc is no longer accessible, so I don't know how much of the rest of my work is original and how much is me remembering his stuff and convincing myself it was my idea all along.