Laurefindel
Legend
Perhaps some automatic up-scaling could be built in. My son played a Skyrim dragonborn-inspired barbarian with Thunderwave as its first level spell. We upscaled it at level 5th and 11th (and presumably 17th if we made it there) to keep it somewhat relevant. So basically, it's a1st level slot at level 1-4, a second level slot at level 5-10, a third level slot at level 11-16, and a fourth level slot at level 17-20.the "trick" is to pick spells that are relevant no mater your level. Misty step is useful at level 3, and still good at level 12. Damage spells would be a mistake (save cantrips, for the reasons you say)
I had a character with Hex as a MI spell that could really have used something like that too. It was far from being OP...