Now we're going to think about the role of Magic in your homebrew world.
Stop for a while and ask yourself:
What's the level of Magic in that world? High or low?
Do all spellcasting classes live there or is anyone missing? Or do lack some specific archetypes?
Are spellcasting classes available for any race or are there any restrictions?
Does Magic function normally or is it different from the typical D&D setting?
Are all spells available or are any of them missing or prohibited?
Are magic items so common that virtually any warrior in an army carries a Sword +1 or are they so rare that even a single potion is a treasure?
Are there magic items shops or where are they commonly found?
Do they function exactly how they are described in DMG or with differences?
Let's how to answer to these questions?
I've seen that many DMs put many sort of limitations to Magic in their setting, both official or homebrew.
Often the real purpose behind those limitations is fear. Fear that PCs can become to powerful with full access to spells and therefore to difficult to "control".
In my younger days I had the same fear and I know what those DMs feel.
But now I know that the real good thing is when a player spellcaster reaches his highest abilities, because he reaches the highest fun.
So be without fear and use few limitations that will be coherent with the flavor of your homebrew world.
Magic and Classes
Allow all spellcasting classes with maybe a couple of limitations on archetypes.
The game has been developed with all classes to be available and to delete one or more could be really unbalancing: imagine a world without Clerics or Druids. Who will heal wounds or cure diseases?
In the Unique World you can find all spellcasting classes but Eldritch Knight. Fighters focus only about fighting and let magic knowledges and studies to wizards. Multiclass Fighter/Wizard is possible.
Warlocks have only Fiend or Great Old One Patrons, and virtually any of them are at some Demons' service.
Magic and Races
On races you can have more limitations with less impacts on the game.
If halflings can't be wizards is not a big problem as long as many other races can be wizards.
Make your choices according to flavor of your homebrew world and write their motivations in couple of lines
In the Unique World:
-Humans can be any spellcasting class but to be a Warlock is considered an abomination.
-Dwarves have strong dislike against Magic because it remembers to them their ancient past of slavery under Dìar's rule. No dwarf will ever be Wizard or Bard or Warlock. Sorcerer dwarves are viewed as abominations and killed by clanmembers when they show the first signs of their powers. Spellcasting Dwarves are only Clerics of the Divine Principle.
-Halflings won't ever be wizards. They simply lack concentration to study arcane formulas needed for spellcasting. Nor they will ever be Cleric or Druids. They are creatures from another world, without ties with Gaya's divine beings and they won't create them.
Spells and Spellcasting
You can modify something without problems.
To ban some spells doesn't make the game unbalanced since the high number of spells available.
However you should avoid to fully ban a school.
You can add new spells or school or new spellcasting rules; in this case remember the rules of a DM Good Game Designer.
In the Unique World:
- Clerics of the Divine Principle and Druids of the Old Faith can't cast resurrection spells, from Revivify to True Resurrection. None knows why the divine beings don't allow those spells.
- Wizards, Sorcerers and Bards are all members of a group known as the Brotherhood of the Bringers of Light and Knowledge and they must wear a couple of magical bracers, the Bracers of Power. They were created after the Magical Cataclysm to avoid that powerful magical powers will tear apart Gaya again. They block spellcaster to become mad or to change form (who casts spells without Bracers gains ever higher chances to have Infinite Madness or to have physical mutations) and give more versatility and flexibility to spells but causes damage to their bearers if the use them to much (taking inspiration from the Blood Magic rules at dmguild).
You should avoid magical items shops.
One the funniest things of D&D is the satisfaction to find, to earn, a magical item, or maybe to create it, after some adventures.
To buy it is to shrink the magic around it and delete some great opportunities for your campaign's storyline.
I'd allow some tradings with some potions or maybe some scrolls: a good adventurer always needs a couple of potions of healing.
If you want more mistery you can change attunement's rules, maybe avoiding to reveal immediately all item's powers or making them available only with a command word which the player will have to find: investigating, talking with NPC sages or go adventuring.