I'm planing on making my world a Role-playing game inspired universe. For the people living there, it's normal, but for the main character who gets summoned from our world it's a bit... odd. (required TV tropes link: Role-Playing Game Verse - Television Tropes & Idioms )
But, I want to do it in a way that makes sense, I want there to be an explanation for why it's that way.
To start with, I've been working on the magic/technique/ability/whatever system and here's what I have so far;
Everybody has what's called a Mana Core inside them. It's like a spiritual organ or muscle. It continually produces Mana which is the fuel for magic.
The Mana is stored in a person's Ley System, sort of like the circulatory system only with mana instead of blood. Basically it acts like a gas tank for Mana. Once the Ley System is full, the excess Mana is vented into the atmosphere.
Both the Mana Core and Ley System can be damaged or not work properly just like any other parts of a person, however that's not important at this point.
Mana is just the fuel. To actually do something with it you need to use a spell/technique/ability/whatever.
Before using a spell/technique/ability/whatever you have to Realize it.
Realizing a spell means suddenly learning it in a burst of inspiration. Before Realization, a person might not have a clue about magic, while afterwards they can toss fireballs around like it's nothing. Realization can hit anytime, in the middle of a life and death battle, while training, while reading a book, in the middle of household chores, etc. Realization is my explanation for suddenly learning a new spell upon level up without any training. Realization isn't random though, what you get is based off your personality, what you do, and the situation you realize the spell in. Do a lot of fighting and you get a lot of attack spells, chores? You get domestic spells.
Spells tend to have distinct effects, modifying a spell is very hard. A spell that throws a fireball can't be tuned down to light a cigarette. I like the idea of the main character being able to modify his spells, but I don't know how to justify that in-universe... Hacking the spells maybe? Hmm.... I dunno....
-Do stuff, gain XP.
-Realize spell/technique/ability/whatever in a burst of inspiration
-Power spell with mana produced by mana core
-Spell takes effect.
It is possible to do a spell without realizing it provided you have a solid understanding of Magical Theory, but it takes longer and has a risk of going wrong as you have to manually go through the steps of the spell that were automated during Realization. Luckily manually casting a spell is one of the fastest and most reliable ways to then Realize that particular spell. A mark of a great mage is manually developing and then realizing their own spell.
Spells can also be inscribed/imbuded in objects, but again it's not as effective as a realized spell. That said, certain Artifacts found in Dungeons have Realized spells of their own.