When I first learned about Mana-based, that is, spell-point-based, magic systems I was all agog. Here finally was the organic-feeling, sensible limitation on magic that I craved, as I'd never been able to agree with the 'Vancian' paradigm of D&D magic. Then I ran into a wall.
My personal preference for dark, unpredictable and dangerous magic was not served too well by spell-point systems. Magic in such systems started to feel more scientific than mythic to me. I still liked the underlying idea of tracking how much energy you'd used, but I wanted a different paradigm. Now my subconscious is apparently some kind of closet bulldog, as it seems to like chewing on old ideas and questions I'd forgotten I ever asked, since this suddenly rose out of the murky depths of too much coffee and having to do one thing while wanting to do something else.
The gist: a system of freeform magic powered by accumulation of a randomized quantity of spiritual Debt, and discharged in random quantities also by acts of Submission. My preferences for pulp-style sword and sorcery made my treatment of this rather dark, but it can still work in a high-fantasy game where good/evil are more balanced. As is, this'll reek quite a bit of Howard, Moorcock, Lovecraft and Ashton-Smith ...
All magic requires a Patron to effect it. Mankind has no magic. Ergo, to have magic happen you must get something supernatural/extraplanar to make it happen for you. Sorcerers may have multiple Patrons, as each one has its own brand of magic.
Spells are called Invocations in this system, as you're calling on Something/Someone for your magic. Invocations are freeform; tell the GM what you want to happen, the GM warns you how many dice of Debt you'll get for it, decide whether you want to go ahead or not, and if you go ahead, make your roll to see if you get what you wanted. If you succeed, what you wanted to happen does so. If you fail, something unintended happens. For example if you wanted to summon a demon to devour your foes but blew your roll, the demon appears and makes a beeline for the fried chicken on the table.
Making an Invocation incurs Debt. The GM determines how many six-sided dice he's going to roll for your Debt; the roll should be open, and you should know before you commit to the Invocation how many dice the GM intended to roll. The result is added to your current Debt, if any.
You have a Debt Threshold; if you're using the 3-18 D&D stat scale, this could be INT + WIS + CON. As you accumulate Debt, you feel increasing psychic pressure to do something about it from your Patron/s. Your Threshold is how much Debt you can acccumulate before this psychic pressure grows too great for you to bear. If you go over it, Bad Things Happen: your Patron may show up in a bad temper, your spells may backfire, or worst of all, your Patron cuts you off for a while and you find yourself unable to work magic.
Debt is discharged by making acts of Submission to your Patron. These acts depend on who and what your Patron is. Submission may require offering blood sacrifices and participating in Grisly RitesTM, or if you Patron happens to be a holy type, meditation/prayer and doing Good DeedsTM. An entire quest could be made just to discharge a huge Debt. The more extreme the act of Submission is, the more Debt it can erase.
One way to discharge Debt is to study the Grimoire/s in your possession. Grimoires in this sytem are demon-inspired writings -- scrolls, books, ancient tomb inscriptions, and the notes of earlier researchers -- that name demons and discuss their natures. They're also vicious-cycle traps. To learn magic, read the Grimoire. But as you do magic, you need to keep studying the Grimoire for fresh insights; the demons who got that Grimoire written intended it this way, for as you understand a Grimoire better your mind becomes more and more like that of a demon, until you either crack, as most Grimoire owners do, or become a pawn of the demon.
One of the things you can ask for with a successful Invocation is a magical Gift; an item, or an ability, that you can use at will though usually for a limited time or number of uses. The greater the power and permanency of the Gift, the greater the Debt you'll accrue for it. For example, you could ask for a talisman that protects you against all iron weapons; this is worth a lot of Debt. It would be worth even more if it protected you vs. all weapons.
Examples of appropriate Gifts include: a Mesmeric Gaze, Regeneration, Animal Speech/Animal Command, Immunity to something, enchanting a weapon, and so on.
Another way of handling the Debt idea is to track the Favor of your Patrons instead. Favor can be positive, which means your patron powers kinda owe you, or zero to negative, which means any further magic increases the magnitude and urgency of what you must do to gain Favor back. For example, you had 5 Favor, but cast a spell that ended up costing 15; now your Favor is at 5 -15 = -10.
What's the benefit of positive Favor? Maybe faster access to magic? If your Favor is zero to negative, you may have to spend a round or more bargaining with your Patrons to get what you want.