Spellburn in DCC is pretty badass, although the existing system requires the spellcheck to make sense beyond recovering spent spells, making using it in LotFP tricky.
What if it wasn't optional?
Magic Users no longer memorise spells, but they do have to lug either a spellbook or a scroll containing spells they wish to cast around with them. Casting a spell now burns off ability points from a physical statistic (determined by spell or a roll) equal to the spells level.
Using the same level brackets as exist for memorisation, we can determine what is within the wizards easy grasp - second level spells become usable at third level, for example. Magic Users can over-extend, but they burn DOUBLE the spells level in ability points, meaning second level spells cost four ability points until the caster reaches third level, third level spells cost six until fifth level, etc. Spellburn damage must heal naturally, at the rate of about a point every [whenever makes sense for your campaign. I'd go for a week.]
To make magic a little more palatable, Magic Users do slowly develop a 'Buffer' of faux-ability points to feed the magic, representing their finer grasp of the magical arts as they advance up the levels. The Buffer should probably regenerate daily, and is equal to the Magic Users level. This amount never exceeds the cost for over-extension, meaning doing so will always incur at least a small amount of spellburn.
Other fun things to steal from DCC for Magic Users is the magic corruption table, perhaps to be invoked whenever a novice warlock over-extends? Or just as a level-up 'perk.' Also Patrons would be really cool. Even if you never intend on running DCC, it's worth picking up just for subsystems to cannibalize.