It's a neat idea. Clearly, it would be better served by being built into the system but I've never let that stop me before.
Half-baked idea number one: D&D already has schools of magic for every spell so make the rule "to add a spell to your spellbook you must have another spell already in your spellbook that is from the same school and is at least half the level of the spell you are adding". Using the round down rules of 5e that means no requirement for any 1st level spell, at least one 1st level spell before you can add a 2nd level spell, at least one 2nd level spell before you can add a 4th, ..., and at least one 4th level spell before you can add a 9th. Not difficult but does meet the requirement of needing previous training in a school to master higher level spells. You can adjust this to be 2/3, one level lower, or whatever tickles your fancy. This rule wouldn't apply Clerics, Druids, Rangers, and Paladins since they can adjust their spell lists whenever. Perhaps Sorcerers are special because they get to ignore it too?
Half-baked idea number two: If memory serves Morrus' Level Up gives a bunch of "tags" to spells like Divine, Fire, Nature, Water, Arcane, etc. It would be pretty easy to make highly customizable chains from that. Every time you get a new spell level you just pick another spell on your class list that must include your chosen tag. Since many of the spells have multiple tags you could easily have two characters with the same tag that end up with entirely different spells.