One thing I would change is to give all spellcasters osirions which scale with level. This would allow a wizard to still cast some "minor" spells when he burned all his normal spell slots.
... I still working on some rules here, but can't find anything which fits yet; and I don't have a round to test them at the moment :(
At the risk of committing unspeakable heresy, 5th Edition not only has unlimited cantrips, the damaging ones also scale in damage two or three times over your career.
Read about this but don't have the rules at hand, but maybe worth some salvaging :D