Is the magic system for mages from Monte Cook's World of Darkness workable in 5e?
It's described in MCWoD, p. 173.
I can't quote it verbatim at this point, but basically, every level you get a certain number of magic component points, which you spend on the components of your spell. At level 1 it's 60 points, and at 20 it's like 380. It takes more than two days sleep to recover all your points (5% per hour spent resting), and spell costs add up pretty fast.
All spells have Area, Duration, and Range components, then you add in additional components to match what you want your effect to do. Additional components are similar to the schools of magic, so like Energy, Conjure, Move, Heal, Divine, etc, plus Metamagic as a component (Damage is a component, without it, your Energy based fire does no damage). So you could use the Energy component to create fire, for example. So you could spend 2 points on Range (Touch), 0 on Duration (instant), 1 on area (single target), 1 on Energy (to make fire), 2 on Damage (for 2d6 hit points of damage), one on Metaspell (1 standard action casting time) and cast a fire-based touch attack for a cost of 7 points.
You gain more points as you level, and you must have a full rest to recover points. Each spell casting takes a full round action unless you use Metamagic components (as above) to change that. (Adjacent enemies get an attack of opportunity) then make a Spellcasting skill check, difficulty of the spell's cost plus any Exhaustion points you have. Once you have determined that it casts successfully, you spend your magic points equal to the cost. Then, add any exhaustion rating on the spell to your current exhaustion points. Finally, the spell takes effect.
Exhaustion comes from casting harder spells. So you take your mage level plus 10, and spells that go above that number in component costs give you exhaustion points equal to the excess. So if you're lv 5 and cast a spell that costs 20 component points, you will gain 5 exhaustion points.
This isn't comprehensive, and I'm pretty sure the costs scale up, so your lv 1 mage isn't throwing all in for a 55d6 fireball or anything.
Spellcasting is just a d20 plus you Spellcraft vs the target's DC.
This system pretty much turns all the caster classes into one single magic-user class with flavor based on what they prefer to do. Plus, not everyone has access to all the components either.