Even in the old lore, Ryze's story was pretty isolated. Thematically, he's a guy that knows lots of magic with a big scroll and some tattoos. He had a somewhat minor role in League, but Idk what they'll do with him now. He's a rival of Brand's, so another ice champ?
I think Ryze has the least to worry about in the lore department honestly. He's a mage who is trying to protect the most dangerous magics of Runeterra. I can imagine a lot of people would want whatever is in that scroll on his back, and perhaps that scroll does relate to Brand.
and 
may have an interest in learning of its secrets as well.
Well Ryze has always been the mysterious and lone wolf type of guy, if I was to create his background then I would say that he was born in Shurima. and wandered the world a lot and eventually learned of and improved upon the ascension magic that created along with the knowledge from other teachers, who included his closest/implied lover lilith who he stole the scroll on his back from to protect it from her and others, as he decided to proclaim himself the protector of magic, He would then transform himself accidentally into his current form form his phenomenal mastery of magic and self disciple and would then continue to protect people from dangerous magic to people. Ryze in his backstory have many enemies and few allies, and along with being on the run for so long has left him bitter and angry as he is wanted man that people would want the magical knowledge that he possesses, kill him for theft, for use him for their own dark ends while spreading lies about him being a madman that drains magic and life force of people.
Allies:
: for being his first teacher and leaving a great impact on his views on magic and how it destroyed Shurima. Ryze has a close friendship with Nasus.
: Zillian has great knowledge of temporal magic and Ryze also seeks to help him cure himself of his chrono-displasia as gratitude for teaching him.
: Both are seek to protect Runeterra form world ending threats, Ryze terrestrial, Kassadin form extraterrestrial ones and both are underground as they are both seen by the outside world as madmen. so the two have an alliance with each other.
Rival's:
:Ryze knows all about the Black rose and the Black rose knows all about Ryze. Because of his knowledge of each other's darkest secrets. Emilia and Ryze are sworn enemies as Ryze seeks to destroy the Black rose society for their use of nefarious magic and Leblanc wants Ryze's Scroll to their own ends.
: Lissandra is to Ryze what the void is to Kassadin, the closest threat to Runeterra that is right under peoples noses. As with LeBlanc, Ryze know of Lissandra's secret and of her plan to summon the Frozen Watchers back to Runeterra. but he has little proof of her involvement with them and the means to expose to the rest of the Freljord
: Xerath is Ryze's Anti-Idol. Ryze's was inspired by the tell of Xerath's ruinous attempt of ascension and the horrors that it entailed. Ironically Ryze through great mastery become what Xerath might have become should his ascension have gone smoothly. Now that Xerath is Free. Ryze is gunning to take Xerath down, once and for all.
Backstory:
While there are many kinds of sorcerers and arcanists, Ryze is one of a kind. Rumored to be one of the most powerful magical being on Runeterra Many would have been eager to learn form the best of the best. However Ryze is also a Spellsteal. having stolen and even killed many magi to prevent them from using magic that was dark or to stop them from their reckless use of magic in an attempt to avert the coming of another rune war, Or something even worse. As a result of his bold actions he was labeled a magic and a life eater. Always on the run and traveling incognito to evade capture.
Ryze was born in the sands or Shurima and was a curious child, having a strong wanderlust and affinity for magic, the latter something that the natives or Shurima feared as it was the cause to the ruin of their once great civilization. A young Ryze was ostracized from other children and never felt as a member of his tribe. One day while traveling along, he accidentally stumbled into the lair of Nasus, the former Curator of Ancient Shurima. Wanting to learn more about magic and history, he pestered the ancient being enough that he finally relented. as he learned more about the past, their bound grew, like father and son, Nasus revealed the tragedy that led to the ruin of Shurima. Ryze was shocked and finally understanding the reason for his people fear of him. When He was ready to leave his mentor, Nasus made Ryze Swear to never use magic or reckless or selfish ends and always be responsible when using it. This lead to Ryze's decision to become a protector of forbidden magic to prevent anything similar from ever happening again. Returning to prove his tribesmen that he finally could control magic and there was nothing to fear, His kinsmen were horrified if his progress, and Ryze was forced to flee but not before kill a few of his kinsmen including some of his own family members to escape.
Left bitter and believing that people would always seek their own demise subconsciously. he decided to travel the world to learn other kinds of magic to and to stop abusers of magic from ever harming others. During one of his Travels he meet a tattooed woman named Lilith, with whom he gained his Iconic tattoo's form to harness Thorn magic, the magic she had mastered. While Nasus thought him about own magic power increased them, it was Lilith that helped Ryze to refine his skills as a mage that would eventually become his fighting style. Eventually Lilith revealed to Ryze a Scroll of Great power and the history of it, a Scroll of such great magic that he began tremble with fear. While Ryze had a close bound her, even sharing more intimate knowledge with each other, Ryze know that Lilith was somewhat tempted to use the scroll for her own ends and may even have used the scroll in the past. Ryze made the second hardest choose in his life. He steal the legendary Scroll, which content shall never been know to any other and left his beloved mentor in the dead of night while she was asleep with as heavy a heart as on the day of the killing of his tribesmen.
And so he began his quest.
Overall I think Ryze being some sort of wanderer who doesn't associate with any nation is kinda ok,I mean there aren't really many other champs like that,at least no human ones,so that is a kind of unique spot for ryze.
Its also fitting considering that Ryze is probably one of the most iconic LoL-characters(or at least he used to be this),giving him a more neutral standpoint.
Though I think they'd need to expand upon his lore. I would like to know where he'd came from,where he got his knowledge about magic and that scroll(his judgement did tell us about the scroll,but it might not stay canon). If Ryze is from one of the current nations,I could actually imagine him being a former noxian.I mean Ryze has a lot of muscles and I think it would be fitting for noxus that even their mages are physically trained.Of course there are also the voodoo lands who would kinda fit that too. However him being "immortal"(as in not aging) and being from an extinct faction could also be a possibility.
