One thing that I've wanted to do for some time is adapt the system from the "Paths of Power" article from Dragon #216 into 5e. The basically idea was that all the wizard spells (for AD&D 2e) were arranged in paths of conceptually related spells rather than schools. The paths had somewhat evocative names and contained ten or so spells (though some were bigger than others). For instance, the Scabrous Road contained the spells chill touch, choke, ghoul touch, spectral hand, mummy touch, pain touch, vampiric touch, paralyze, mummy rot, ghoul gauntlet, lich touch, and energy drain—basically, all the necromancer touch attack spells. You had a limited number of paths you could learn from, but if you knew every spell on a path that you could cast at your level, it didn't count against that limit. It naturally leads to more focused, thematic casters and invites interesting worldbuilding ideas, like secret paths known only to certain teachers or groups, or rival schools featuring practitioners of different paths of magic. It always seemed to me a good way to moderate spellcaster power by restricting access to spells without feeling arbitrary—maybe you can learn silvery barbs, but you can't get it without losing access to shield and feather fall, etc.