I would probably go with the themes of blood and corruption.
Having slept on it, here's a draft for y'all:
Corruption Points
Each time you cast a spell, you gain corruption points equal to the spell's level. Cantrips count as half your level, rounded down.
Yes, this means level 1 casters cast cantrips for free.
You have a corruption threshold that increases as you level up. For a quick and dirty idea of appropriate thresholds, use your maximum spell level squared. At level 1, your corruption threshold is... 1. At level 9, you can cast level 5 spells so it is 25.
Taking a short rest reduces your corruption total by your level. Taking a long rest resets it to zero, assuming you were able to meditate, pray or cleanse during the night.
Accumulating corruption is risk-free until you exceed your threshold. Then you must immediately roll a d20 adding the surplus corruption and face the consequences:
[Add table here with various S&S themed results. If there are changes to the environment, it's okay not to give the caster a way to prevent this. This should represent the risk of the caster getting mistrusted by villagers or thrown out of town. More direct results like twisted limbs or the spell being redirected to the wrong target should allow for a saving throw]
For instance, if Akiro the 5th level Wizard have 10 Corruption and casts a third level spell, his new total is 13, which triggers a roll on the Corruption Table since his corruption threshold is 9. To the d20 we add 13-9, so d20+4. If we roll 19, thus getting a result of greater than 20, that should definitely be outright crippling, I suggest permanently. Maybe we can entice the player of Akiro to turn to the dark side (see what I did there

) by offering him a save by transferring the crippling curse onto a hapless bystander, say a beautiful warrior named Valeria...
If you want you can even skip keeping track of spell slots, using the above as a variant spell point system.
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Dev notes:
The intent is to allow a caster to be able to use up roughly half of the daily allotment of the official spell point variant, and then have to take two short rests to get access to the other half. Without risking corruption rolls, that is. I know it doesn't come across from the above, but that corruption results table is definitely intended to be bad news. It's not like the Wild Magic Sorcerer which is built on the assumption you
want to roll on its table!
I know that's not exactly what the above simplified system results in, but I didn't like the idea to actually use the spell point numbers in a first draft like this since I feel those are overly complicated (hard to remember). I mostly wrote it up quickly to get the idea across, not present it as a carefully calibrated publication-ready submission
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Blood Points
You can supercharge a spell by accruing Blood Points instead of Corruption Points. (An equal amount)
Each time you get to cast the spell at one spell level higher than your maximum and with +2 to your DC.
Blood points are summed together with your corruption points when comparing to your corruption threshold.
The difference is that it's far harder to get rid of blood points. Assuming you run episodic adventures (rather than counting exact days during downtime), the GM might let you halve your total at the end of each adventure. At the heroic end of the scale, you might be allowed to remove 1 blood point for each long rest, though this is likely too good if there's long uneventful journeys or long stretches of downtime where you count the days between scenarios.
Dev notes: The intent here is to compensate casters for the reduced variety in spells by allowing them to supercharge their casting for the cost of increased risk of corruption.
Do note that if HP totals are reduced, this might not be necessary assuming the caster still has access to some multi-target damage spells.
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As a final note, both corruption and blood points should be possible to transfer elsewhere - having somebody else pay the cost, to explain why most sorcerers are dark and evil in this setting.
For corruption, this is probably best handled when resolving Corruption table rolls, rather than actually tracking corruption points injected into NPCs. (See example above).
For blood points, simply say you can substitute blood points for hp damage
and an equal reduction in maximum hit points. (Paying with actual Constitution points isn't appropriate for 5E the way it could work in 3E). The easiest approach is to say this damage can't be healed (short of Regenerate spells etc), since the point isn't to boost player character casters, but to lure them to (ab)use NPCs.
It is when the game mechanics explain why villains drag along sacrificial victims you have a rule set that supports S&S.
Also note that there is no rule such as "if you do this too much your alignment shifts to Evil". In S&S, your actions might come to haunt you later, or they might not. But that's between the GM and the player, and not for the game rules to moralize over.
Cheers