CapnZapp
Legend
First question: are you prepared to give up D&Ds idea that everybody should be (roughly) equally capable to contribute in combat?So I'm reading over Beyond The Wall, and I'm quickly falling in love with the magic system.
For the uninitiated, a mage has access to 3 types of magic.
Cantrips - mostly minor magics. These require an ability check or they can backfire.
Spells - mostly equivalent to 5e 1st and 2nd level spells. There are no levels to these. You can merely cast a number of spells per day equal to your level.
Rituals - these are more powrful magics that require exotic material components and takes an hour per level of the ritual to cast. These are more equivalent to 3rd level and beyond.
Rituals are wheee it gets really interesting. They have a range of magic such as raising undead, resurrection and causing a common shared dream in a community, but these take nearly a full day to cast. I love this as a mechanism. It allows mages to be powerful without the idea that they can cast wish for breakfast each morning.
How could this be implemented in 5e, if you were to do so?
If you are, easy - just use this on top of the existing full caster chassi.
If you aren't, also easy, accept that Wizard becomes a NPC class and give this to half casters like Rangers and Paladins (and maybe even one thirds casters like Eldritch Knights and Arcane Tricksters).