I've got a small theory about this.
In her original lore, 
was regarded as a gifted child able to learn any spell. In the most recent one though, she no longer is, but then, how does Elementalist Lux come to happen in the future? Lux seemed to only be gifted for light.
From here comes different options
1.She learned with the mages to use more elements, so learning them is possible and shouldn't be that rare in Runeterra, but then what is arcane magics, that is only used by very powerfull magicians like 
and 
? . Does that mean that Arcane is inferior to the rest of elements? so those 2 would be inferior to or 
. If arcane was to be superior , Why can't use any other element?
2-You have to be born with that element, so a X factor allowed her to learn the rest of elemental magics.
3.Something can help a normal human become a mage by giving it an elemental affinity
This is where the theory starts.
Reading the story of both and 
. I can't help but think that the World Runes are the origin not only of life, but magic itself, both because Ryze knows how destructive they can be, and because the dragons take their fiery looks and powers of a small rune fragment embedded in their hearts.
I think then, that magics, and every element of those come from fragments of Runes or the original World runes themselves, and that those magic champions got in contact with them in some undertermined time on their life.
came in contact with the Light Rune, maybe in some of her travels with her family
came in contact with the Earth Rune, maybe when she got lost in ancient ruins in the middle of Shurima
Vlad's master possessed a fragment of the Blood Rune inside him, just by living alongside it, he started to awaken the blood magic, but his power wasn't complete until he took the fragment from the bloodstream of his master and got it inside him.
got his powers from the Shadow Rune, kept inside a box because of its dangerous characteristics as a World Rune with untinkable power. The Shadow Rune might reside right now inside him, and just by going to his dojo, his disciples start to awaken to the Shadows, but aren't as strong if they leave early (like what might have happened to 
if she was once his disciple).
We haven't seen Zed's true face yet, he could be just a living shadow with pulsating white lines (That define a shadow master, as revealed earlier by Riot), being these lines power marks created by being overexposed to the power of that Rune.
is incredibly strong, but he doesn't posess any Rune or fragment inside him, he manages to supply this with Rhaast's amazing power when in Shadow Assassin form, that boosts his already strong Shadow magic beyond the limits.
the fire sealed on ice that got inside him WAS the Fire Rune, now he is the embodiment of Fire
's master might have posessed the Wind Rune or a fragment of it, but only allowed the best warriors among its students to get close enought to said Rune to start developing Wind Magic. When the War started, someone knew that the Rune/fragment was there, so killing the elder and getting such powerful artifact was vital
He has come in contact with so many different elemental Runes and fragments that he can no longer use different elements, they all converged into unity, a perfect and pure element, called "Arcane"
This is just a mad theory though, but i think it would fit with all the points exposed earlier. Why mages are so rare? Why does someone born with the Light magic becomes an elementalist? why isn't it any more? What are the creatures that come to fight her and destroy Demacia? These would probably be creatures mutated or born of the World Runes, causing them to become ultra powerful and wild beings, but could also be magic feeding beasts that ravage Demacia in order to find the Rune fragments or World Runes that Lux is hiding there.
?
