Help with Magic in Modern World

[Royolis]

I've read over the what should belong in the World Building forum, and I believe from what I'm about to ask, this belongs here. Sorry if I was mislead and posted it in the wrong section in advance.

I've been lurking around for a while looking at homebrew stuff and like, and thought the community here can help me with my problem. I'm going to be running a D&D 5e campaign in a modern-world setting (low-magic), however, the world use to be full of magic (high-magic). There still exists magic in the world, but most (almost all) humans are completely oblivious to its existence or think of it as myth and legend (like Arthurian legend). Races that were inherently magical have become reclusive and highly xenophobic (if not extinct), natural healing springs and the like have become tourist traps, and items that had magical enchantments have either rusted away to nothingness, become heavily guarded, or are family heirlooms and are hanging above the fireplace.

I've got most of the world hashed out, possible friends and foes, places, plot, etc. The one thing that's been bugging me though is the idea of allowing people to use unaltered magical classes. Now, I know one way of solving the whole problem is to not allow magic-using classes, but I find that to be a little bland considering the world that they're going to play in. It's very unlikely that a human would stumble into magic and learn how to use it, but it's not impossible.

So, without removing magic using classes from the game, how should I change those classes, or if I even should change them?

Some of the races I don't plan on changing the current classes for, like Dragonkind and Elves; they're two of the races that have become withdrawn from humanity to protect their lives and/or culture. Therefore, they would still have the ability to teach magic to each other in an isolated environment.

I've also thought of a few ways to allow humans and others to gain access to magic:

1. Exclude Bard and Wizard, but allow Cleric, Druid, Sorcerer, and Warlock.

Part of my rational behind this is that Bard and Wizard learn through either books or colleges, while the others learn from either higher beings, nature, or is passed down through a bloodline. Being so removed from magic, I kinda feel that the only way to relearn of magic for short-lifespan races would be through a higher power, religion, shamanistic rituals, pacts, etc. Sorcerers are kinda on the fence though, but things have been known to skip generations.

2. Half or 3/4 spell slots (rounding up), consequences for casting 6th level and higher, but able to regain spells after 4 hour rest.

I realize that half would likely mean only 9 spell slots for most classes, which is awful but fitting. The consequences would be as follows:
6th Level: Half movement for the turn. If you moved before casting, then half movement next turn.
7th Level: No movement for the turn. 1 points of exhaustion. Same movement conditional as 6th level.
8th Level: DC 10 Con save to not faint. If they do faint, they use their next turn waking back up. 2 points of exhaustion.
9th Level: In addition to the other possible side effects (I know Wish has some), DC 15 Con save not to fall unconscious. On a fail, must make a DC 15 Con save to become conscious. 3 points of exhaustion.

Now that I look back on it, the regain spell slot thing seems OP, but I feel like nobody would even look at a human spell caster if I didn't do something to curve the negatives.

I know I might be very much over complicating this, but I want to give my players the best range of possibilities as possible. I'm just looking for some advice on how I should go about this, if I'm on the right track, or if I should even use 5e to accomplish this. Thank you.

[Zozma]

5E, like most modern tabletops, was built with balance in mind. Magic-using classes already have weaknesses included to balance out the fact that they have spell slots, so limiting them further will throw off the game's equilibrium, and any attempt at rebalancing them (with your quicker slot regeneration, for example) might push things too far in one direction or another. But that's just me pointing out the obvious.

I'm not sure limiting casters in your setting is necessary. From the sound of it there's still magic around, but humans have simply forgotten how to use it and can presumably rediscover how to access it. That's pretty easy to simulate with the built in leveling system. If you really want there to be an increased cost, I'd include it roleplaying, quest giving, and storytelling; it sounds like most of your casters get their spells from an outside source, so maybe make it so those sources expect payment with interest. Make magic have a price that's not reflected in its actual mechanics. That keeps balance from being an overt concern, and having real world consequences reflected in players' choices will make it feel like you're broadening prospects rather than limiting them.

As for wizards or bards, I'd always recommend changing the fluff of something before changing its mechanics or locking it out entirely. There aren't going to be any scruffy wizards teaching one another spells in big towers, but it doesn't sound out of the question in your setting for an eccentric academic to stumble upon an old spellbook, which is enough to start them out as a level one wizard. Alternatively, have wizards and bards require a source of power like warlocks or clerics: latent magical nodes, celestial phenomena, or an actual patron.