Hello everybody. I'm going to be running my first campaign here in about a week and I am considering a mechanic to make magic a little more interesting when it comes to leveling up and unlocking new spells.

One of the premises of my campaign setting is that the old empire (now fallen in a horrendous cataclysm) was so powerful because it had unlocked magical might and secrets that the current nations no longer have access to. This is going to turn into all manner of interesting story elements as i intend to have that be a really big deal, with kings and scholars all hoping to recapture the lost might of the old world and so on.

What I would like to do is to translate this concept of powerful magics being lost into the game in terms of mechanics as well. What this means to me is not simply handing out spells every time my casters level up. One of my favorite mechanics in the game that is a part of the basic ruleset already is the wizard searching for and collecting spells and having to place them in his book to use them. I think this adds to the flavor of a wizard very well and keeps a player constantly searching for new spells and being very excited when they find one in a treasure hoard or for sale at a local magic emporium or what have you. That being said, even the wizard gets a few spells just for free every time they gain a new level. I'm not sure I want to keep that mechanic, at least for higher levels where magic of that power is rare or nonexistent in the setting.

My idea is this. When a spell caster levels up to unlock new magics of lower levels (cantrips through 5th, maybe 6th level) there is no change to the mechanics. They can pick from their spell list if they are a wizard/sorcerer/warlock/ranger/bard or just unlock those spells to choose at the top of the day as a paladin/druid/cleric. However, when the PC's begin to gain access to spells of 6th (maybe 7th) level or higher, they may unlock those spell slots to use, but cannot simply add a 6th level spell to their list or prepare them for the day. (e.g. an 11th level wizard could cast fireball as a 6th level spell, but could not just choose to add disintegrate to their spell book).

Now of course, I still want high level magic in the game and it does exist. It just isn't free. If a wizard wants to cast Disintegrate, or Teleport or Control Weather, they will have to either find some scholar sage at the magical university to teach it to them, persuade an emperor to allow them access to their vault of ancient tomes in which secrets of old magic are recorded, find a book of secrets in a dragons hoard, or some other equally interesting thing. A sorcerer or bard would have to do the same thing.

A Cleric, Warlock, or Druid presents a different problem. They don't learn their spells in the same way the other classes do. A Cleric is gifted divine magic by their deity, a Warlock only gains access to spells of 6th level or higher as a gift from their patron, and druids derive their magic from their connection to nature and the magical energies that suffuse it. For these classes I'm thinking I would make the clerics spend time in special fasting and prayer to their deity and be given some holy quest to prove their worth and capability to wield such powerful abilities to their god/goddess before they could unlock those spells. A warlock would have to go through a similar ordeal as decided by their patron. Druids, I'm less sure about. Perhaps I could make them search out some ancient arch-druid or animal/earth spirit who could gift/teach them the ability to shape nature on that level.

Tl;dr In my setting, magics above 5th level are exceptionally rare and I want to reflect that in the game mechanics by making my players have to go through quests or ordeals to unlock higher level magical abilities.

So I'm hoping that the subreddit community here could help me with a few things. One, I just want opinions from players and DM's perspectives alike. What do you think of my proposed mechanic? Would you love it and think it's very interesting and thematic? Would it be tedious and frustrating to have to dole out spells to the players in this way or to have to collect them as a player? Am I unfairly nerfing casters, or giving them to much prominence in the setting relative to the melee classes? Should a first timer like me just stay away from messing with the rules to this extent and keep things simple? Two, If you do like the mechanic and think I should give it a shot, I would love any advice on polishing it up. how should I flavor the fact that druids/clerics/bards have to earn their high level spells from some source or other? What would be ways of giving them quests/ordeals that are difficult and interesting, but not so time consuming that they are constantly taking the players away from what they want to do so they can do what they have to do?

Thanks in advance! All opinions and advice are greatly appreciated!