The difficulty I have, in system after system, is to make magic magical, and to make it perilous and difficult without becoming uselessly dangerous to the user.
D&D magic, if used by the book, is really rather mundane. It is virtually omnipresent; it might as well be modern or sci-fi technology. At best, it makes the game run like a superhero RPG. The latest iterations of D&D (the 4th and 5th editions) are the most egregious in this respect, with virtually every character class tossing magic around with gay abandon, but it's a feature of the system that has been building right from the beginning back in the '70s.
In the Microlite D20 version of D&D3e, there are no spell slots as such. Spells cost the character Hit-Points to cast. I don't recall the exact ratio, but it's probably 1 hp per spell level. That strikes me as a useful mechanism to control the willy-nilly flinging about of cantrips like Light and such-like.
I'm thinking of instituting a casting cost of, say, 1d6 + 1 hit-point per spell level to cast, and half that (rounded down) for each additional round of maintenance, With the large number of hit-points most characters have in 5th Edition, along with the ease and speed of recovery, I don't think it would be too onerous a price to pay, while still resulting in a noticeable diminishing of resources over time. It would make items for creating spell-like effects a lot more desirable, and perhaps magical energy reserves (like IOUN stones or something) could be added.
This does nothing much to address the mundane omnipresence of magic in D&D, but one thing at a time.
This guy includes in his Generic Adventurer rules just the sort of thing I mean:
"No more memorisation, you know all the spells you know.Lamentations characters have far fewer hit-points than D&D5e characters, so I'd stick with the additional d6 hp on casting. I do like the idea that spell-casting wear and tear comes off the character's hit-point maximum rather than their current total. Though I'm not quite sure what he means by "allows magic users to not stop casting spell from fear of killing themselves". I also like the idea of a magic-user being able to cast spells too high for their own level by paying penalty rates, but I think that idea needs a little more attention.
To cast a spell, lose maximum HP equal to the spell's level. Maximum as in off the top, not the total. Your total HP will reduce if you max HP drops below it. (this prevents just healing yourself to ignore the difficulty of magic and also allows magic users to not stop casting spell from fear of killing themselves)
If a spell would be unavailable to a magic user of your level (according to default Lamentations rules) it costs double. Example: A level 4 character casting a level 3 spell (not normally available until level 5) would lose 6 max HP when casting it
Your max HP recover after 8 hours rest"