So, if you're casting a cantrip, your initiative score is reduced by 1d6. If you want to cast a level three spell, your initiative count would be 1d6+3 less.
If you take damage, or have to endure some sort of environmental hardship before your initiative comes up, you must make a Concentration save to be able to cast the spell. If you fail, your action is used, and the spell is not cast (though the spell slot, nor any material components, are not used up). You can still move and take reactions and bonus actions if appropriate.
Rituals : casting time one hour per total levels plus 1d4 hours.For example a level 4 ritual would take 1+2+3+4=10+1d4 hours to cast. Note that partaking in a ritual, either as caster or recipient, does NOT count as resting.
Corruption : casting without a patron or via ritual may result in corruption.
Note: I haven't really thought this one completely through, and I'm not sure quite how workable it would be. I really just want some mechanism that might encourage people to consider before using magic willy-nilly.Channelling magical forces directly through one's own body, without the buffer created by a ritual or magical intermediary, is potentially hazardous to one's spiritual well-being. There are certain physical signs of magical corruption, but more important are the mental effects: gradually (if it's not already there) the magic-user's alignment shifts around towards chaos and evil.
There are rituals designed to remove magical corruption, or to transfer it to some other unfortunate.
When you cast any spell, make a DC 5 + spell level save vs. your magic-using characteristic. If you fail, you gain one point of Corruption. I'll let you know what that means as and when it occurs.
Note: Warlocks, Clerics, and Paladins are deemed to get their magic through magical intermediaries. Other spell-casters will have to come to their own arrangements if they want a patron.
Note: It occurs to me that this would make an excellent game-mechanical reason for a spell-caster to have a familiar, which would act as a magical conduit as long as it's within line of sight and thus remove the risk of corrupting one's own self with all this magical jiggery-pokery. It would also explain the intimate psychic connection between familiar and "master". I shall therefore Make It So.