A few years ago, I finally developed a good Spell Points system to replace the spell slot system of D&D, but for some reason, it never struck me until the middle of last night that the way to fix the class system of D&D is Feats.
By simply converting all class-based mechanics to Feats, D&D could finally be freed of these irrational limitations.
I would have to rework my Spell Points system to fit a Feats-based D&D, but that's simply a matter of juggling a few numbers.