Freeform Vancian Casting (Whitehack Reigns Supreme)
Magic has been on the minds of plenty of folks across the blogs. Newt's Truth-telling, Bateman's series of great magics to use, Meg's post which could be used for the memorization of spells, Louis's fantastic magic for Barony. Mr. Mann's post Vancian Casting is Good Actually has helped me reframe what I think an enjoyable freeform system that lets Magic Users cast more and punch above their level, while also keeping the threat of death high! High level spells can be downcast as well.
The well read among you will see that I have taken liberally from Whitehack. I also do not suggest or prescribe what fair HP costs for spells should be! Those are decisions you should make for your table to shape the game you want to play.
Magic Users learn spells as per the rules at your table. These spells are now a lense to express power rather than something prescribed. Spells cost HP to cast. A 1st level spell costs 1 HP as a baseline, a 2nd level spell costs 2 HP, and so on. Sleep as written would cost an HP to cast, but if a wizard wanted to empower it or alter it in some way they would negotiate with the referee on what an acceptable HP cost for the spell would be. The MU makes a save vs Spells to halve the damage.
A spell can be forgotten to give a Magic User a d6+ the level of the forgotten spell as HP to spend on a cast. This HP is not added to the MU's health and can only be used to cast the spell they are electing to forget.
Magic Users of the Hero Level may forget all their spells at once to do something that is utterly outside the realm of reality. They will roll a d6 for every slot and take that as immediate damage. They may not save against this damage. If the wizard is killed by this damage they expire after the spell is cast.
A max level Magic User may do this and also Save vs Spells to halve damage.
The usual caveats of Whitehack magic apply. A MU can not use spells to heal themselves nor can they receive health via magical means. They heal twice as quickly as other classes.
This would couple nicely with either cantrip bubbles or a popular cantrip house rule that a wizard has access to cantrips relating to the spells they have prepared.
Examples
Bob the wizard casts Sleep with no empowerment. He takes 1HP of damage and casts the spell.
Putrid the Mangler casts Sleep with the intent of causing a whole inn to pass out save their friends. The GM and player settle on 5 HP worth of damage, Putrid passes the save and the inn goes to bed.
Glob Spitta casts Pass-Wall but wants to make a tunnel big enough through a mountain for an army to pass through. As 7th Level Magic User he has access to 11 spell slots and rolls 11d6. He takes 60 damage and dies. A tunnel big enough for an army cuts through the mountain but Glob vanishes into dust.
Mark the Mercurial wants to cut an alcove in a dungeon for his thief friend to hide in. He downgrades a casting of passwall to just make a 2 foot dent in the corridor for the thief to tuck into, reducing the base cost of the spell from 5HP to 2HP
Considerations
I do not know what your home game is like. I can't judge what a fair cost of HP for a spell is. My advice would be to take whatever spellbook you're using as the baseline and through a healthy discussion come to the total damage.
The no magical means of HP recovery may be too strict! If it is then remove it! But just realize that clerics will be laying on hands to turn the MUs into an even more destructive force.
An important part of this system is that it allows Magic Users to keep spells relevant by upcasting (making more powerful) and to find broader applications for powerful spells by downcasting them. Ideally this system would allow Magic Users to get a few more casts out of their spells, have access to cantrips, and give them some truly mystical powers once they get up there in levels.
Prospective Deliberations
A few more detailed and opinionated examples of how this might go in play might clear up how to approach this in play.
Let's start with Sleep! For all these spells I will be referring to the Delving Deeper spell descriptions.
Spending 1HP would let a Magic User cast the spell as written, 2d6+2 HD creatures affected. But if a player wanted to single target a Gargoyle (4HD) with sleep I would say that's a 6HP cast due to the HD of the creature and guaranteeing a sleep. A low level wizard might be close to dying after a cast like that! But a higher level one would be able to handle it fine enough.
An example of how a downcasting negotiation might go is with Wall of Stone. Instead of casting the full spell maybe a wizard just wants to fill the keyhole of a lock with stone to prevent pursuers from chasing them. Instead of a 5HP cast I would probably declare it as a 1HP cast.
Now for the sacrifice! A magic user wants to forget all their slots to use fireball to detonate a tower filled with powerful turrets and archers. A level 7 MU has 10 spell slots so rolls 10d6 damage! If they live or die that tower is going up in flames, throwing bodies and stone across the battlefield.
All this aims to get more usage out of spells and allow magic users to express that power in ways that are more personal to themselves, while also keeping the root of the Classic Elf Game at its core. Magic Users of a high enough level should be able to sacrifice themselves to do amazing things.