Theory Crafting/Random Idea with spells

[Calen]

This is primarily 5e but the question works for others systems as well I expect.

I am wondering if this idea might work. Inspired by Mass Effect. Spells are used on a cool down basis, there are no spell slots. Instead when using a spell from your prepared list you enter a state of "Mana Depletion" and cannot use magic until that depletion is gone. Each level of the spell cast adds 1 depletion and you lose a depletion at the beginning or end of your turn.
[Side note] Beginning of turn would mean that you couldn't cast reaction spells if you cast a level 1 spell last turn. End of turn would mean that you couldn't cast reaction spells on the following turn.[/Side note]

What are other pros/cons of the system?
Would a class that used this feature need to be a half caster? (No spells over 5th?)
What spells should not be on this class list?
Why is this completely broken and no sane person would conceive of it? :smalltongue:

[RandomPeasant]

This sounds a lot like the Recharge Magic (https://www.d20srd.org/srd/variant/magic/rechargeMagic.htm) variant from 3.5. What that does, basically, is give casters effectively infinite sustain over the course of the day, make them modestly less able to nova in a single fight (which this would amplify, since it sounds like it's shared for all spell levels), and completely break any long-duration buff (and that's with the special handling they added). Overall, I think this is completely fine to use as the basis for a class, even one that gets full casting progression. You just have to make sure they don't get any spell they can repeatedly cast for incremental benefit, which is most utility spells, meaning that they're either pretty useless outside a fight or need some separate abilities to cover that end of things.