Tone of Magic for a Campaign Setting

[CaptainSarathai]

So, I'm getting ready to break away from a group and launch a new campaign meant to be "traditional fantasy" - because that's what I feel D&D should be.
I like Eberron and wierd settings, but I think that even WoC has been getting away from the idea of swords-and-sorcery questing in a realm of fantastic splendor.

So I'm taking the party to Albion, for a sort of "United Kingdom Myths and Legends" setting. It will be a mixture of King Arthur, Celtic, Scottish, and Welsh mythology, a smattering of Norman French, and general tropey medieval things.

The trouble though, is magic. I'm already running a Longer Long Rests variant (Short Rest = 8hrs, Long Rest = 24hrs) but I don't want magic to seem too "easy" as it tends to in traditional D&D. I don't really want to house-rule or ban anything, but how can I downplay arcane magic in order to shine a greater spotlight on Martial and Divine characters?

Merlin is super cool, but Merlin was kinda the only high level Wizard running the show, and was more Gandalf than Harry Potter: magic was cosmically powerful, with a rarity to match, nobody was washing pans with "practical witchcraft"

[Cespenar]

If you don't want houserules, the only thing you can change is the fluff.

So, in your setting, wizards aren't just a class you pick, they are very rare, possibly with extra reasons (magic in the blood, extreme studying, etc.). Speak this with your players and convey the atmosphere you're trying to create. Simply by limiting the amount of magic they'll encounter in the setting, you can create the general effect you require. But coming to a consensus with your players is the only way they can respect (or roleplay respect) their own magic.

[SilverStud]

You're definitely going to have to have a talk with your players. Make sure they understand that you STRONGLY PREFER non-arcane casters. In fact, you'd be perfectly justified in banning warlocks, arcane tricksters, EKs, and wizards. I'd call it a setting decision rather than a house rule.

By banning the arcane casters, you are getting all that lovely messenger-of-God/traditional-druid flavor in your magic!

[Luccan]

Firstly, I would like to point out that limiting just arcane magic won't necessarily limit how "magic" your setting feels. If there were no wizards, sorcs, etc. but you still had 15th level clerics running around, that doesn't solve the issue, its just that a lot more baddies would likely be members of cults (more Conan than King Arthur, really). Just something to keep in mind. That said:

I wouldn't ban all arcane casting or casting in general, if you still want them to encounter it, but unless you make some house rules, it will have to all be choices you and your players consciously make. If you do decide to houserule, there are a few things that spring to mind. You could ban arcane classes and sub classes, relegating their spells to ritual books and things the players can use. You can ban the classes more specifically gifted in that regard, so your EKs and arcane tricksters are ok, but wizards, sorcs, warlocks, and maybe even bards are out. You could require a minimum character level or proficiency with a particular skill.

But, IMO, if you want something more Arthurian, the majority of PCs and NPCs need to have no to low magic classes. A paladin should be one of the party's mightier casters. If you still want a more religious/holy/nonarcane flavor, then the party caster (and there should probably be only one of significant power) should be a cleric or druid. Alternately, consider the Alchemist Artificer from UA and tone down the actual spells, making them more of a man dabbling in arcane arts he (and most others) don't fully understand. Maybe let a player be a sorcerer or pick up a few levels of Wizard down the road. Or better, if you want them to spontaneously discover a random magic talent, they can take that feat that gives them some low level spells. The BBEG can be an arcane though, it fits settings like this. They'd be more of a witch, probably, and Evil (capitalized). And don't forget, magic items will be legends or even if they're insignificant in history, once someone knows you have it, they'll be fearful or jealous.

And as SilverStud said, talk to your players and make sure this is an idea they're cool with. If so, and you think they won't go for arcane casting at all, then you can completely avoid house rules, since they won't play arcanes and you just don't have to populate the world with a few wizards here and there.

[Frogosaurus]

You may want to take a look at Cubicle 7's 5e Middle Earth stuff. It's very low magic, and it's a quintessentially traditional setting.