RPGs! Man this is my jam.
Given it's the last thing I read, I'll address one problem you have first: magic resolution!
There are two systems which come to mind which could potentially work, the first is from Fantasy Flight (the one with the weird dice) whereby a dicepool is made to do actions, such as spellcasting.
The die have different jobs or simulate different circumstances. One of the die types is a success or failure die. Different coloured die have differing sides but the premise is the same - roll more successes than failures and you succeed.
For example the DM thinks this is a difficult spell to cast and gets you to roll 4 die, 2 blue and 2 red. The blue die have 4 success and 2 failures on and the red have 3 and 3.
Other die can be added to an action depending on the situation - for instance, while casting this spell an archer is taking shots at you, the DM will then add another type of die to the pool - the disadvantage (black) die. This die is blank on 3 sides and skull on 3, if you roll a skull then something bad happens regardless of the result. If you roll a failure AND a skull then that amounts to a D&D critical fail. The white advantage die is the opposite, created from a beneficial situation (3 blank 3 + symbols, if + is rolled something good happens even if you roll a failure)
To continue the example - your dicepool is now 5 die 2 blue, 2 red and a black and the chances of something backfiring are pretty high. Note that this system means that it's never (really, really rarely) a case of nothing happening in a turn at all.
This system can be adapted hugely and is great for improvisational play. For your game in particular I can see using d8 instead of d6 to be an idea. You could have each of the eight mana runes on a side and roll the die a number of times equal to your level at the beginning of a round and that's the mana you get to use for that round.
The other system which could work is kind of similar it's called FATE (built from FUDGE, if you know that one) whereby nobody has attributes like str, dex or whatever they just have aspects like:
Alien Outsider if that's your character. It's kind of like a trait system from isometric RPGs.
To do an action you roll against a GM assigned difficulty (like FF above), for example the GM says, this is easy - difficulty 1 and you roll +/- dice and score above 1, you win and can do what you asked to do.
The aspect system comes into play both narratively and mechanically. In narrative, it's what you just are (and can change over time, not relevant now) and mechanically, you can add +2 to rolls if what you're trying to do plays into your aspect.
For example - A piece of strange extra-terran tech has been found at a crash site and you want to identify it GM says difficulty 3 and you roll a total of 1, you can then say - wait a second my man I am an
Alien Outsider, of course I know how to identify interplanetary whadjamacallits, GM says: yeah that's fair okay here's what it is.
In another encounter you shoot at some hobos because they're a little ornery and you're not taking any chances GM rolls a 2 for the Hobos to defend and you roll a 1 to attack and you say - wait a second GM, I'm an
Alien Outsider, so I'm great at shooting! Your GM then says, uh no not really. If you were alien soldier or were using alien weaponry maybe at a push but just being on a foreign planet, no, sorry. So no +2 for you.
I figure the first is better if you want more of a Avatar the Last Airbender feel imo and the second is more for group-based-discussion RP'ing.
Also did you get Wielder from Black Isles fantastic RPG Lionheart: Legacy of the Crusader? Love that game
There's already a system created for pretty much every type of game you could ever want to play. (Including gross armadillo like FATAL, don't even google it.)
When you said about creating a world and it's history I immediately thought again back to one of my favourite internet people Matt Colville with particular reference to worldbuilding, types of players and the like. If you're into creating RPGs it's my humble opinion that you can't go wrong watching all of his videos for guidance. He is first and foremost a game designer, second a writer and third a stellar orator and improvisational artist.