Ryze's scroll contains the horrifying Lady Gragas skin, woe betide the day it is unfurled and the skin bought and used.

Mordekaiser has a crush on Kayle.

Come on? He likes his women heavy and metal. Who else would qualify for that?

Touche

MOrdekaiser is in fact a very hurted guy.
He was a very powerful magician with a beautiful wife, but she left one day, leaving him alone.
He went mad and enchanted an old armor with magical, metal powers, with the hope of releasing his angeron the shadow island´s creature´s, with no sucess, going to the league short after,
He sees in sona the figure of her lovely wife, and thats why he remains most of his time close to her.

Fizz's favorite activity is stalking Vlad. Why? So he can trollpole right over his trollpool every time he does it.

Whole bunch of Nunu head canon:

Nunu was born female. After being raised by Yetis, who don't have much of a gender distinction, she acts like a boy. Being referred to as a boy doesn't bug her at all.

Everybody constantly assumes Willump is Nunu.

Willump gets angrier at being called Nunu than Nunu does at being called a boy.

Nunu is super stoic due to being raised by Yetis and also having so much responsibility at a young age (being the league emissary for Freljord).

Doesn't get along with most people due to being forceful and blunt. Yeti upbringing means Nunu doesn't really care about or understand concepts like subtlety or politeness.

I second someone's earlier suggestion that Nunu and Annie are frenemies. They will eventually grow to become bitter political rivals.

Other:

Sion had children while he was alive. His family line culminated in Dr. Mundo.

Diana wants to kill Leona because she worships the sun, but Leona knows that the sun and moon are both necessary. She really doesn't care that Diana worships the moon and wishes they could just get along.

Lux is looked down upon by Garen because he got to see inside her mind during her judgment, and he saw that she used to hate Demacia. This makes Lux really sad and she wishes she could have a better relationship with her brother.

Urgot wants to die, but he's afraid that if he kills himself, Noxus will just bring him back to life again with an even worse body.

During Riven's estimated 7 years(?) of exile, she traveled all over Valoran and happened to see a lot of awesome things (i.e., Leona's solar flare before she joined the League, Lee Sin setting himself on fire in Ionia, Alistar's gladiatorial fights in Noxus, one of Vi's robberies in Piltover, Skarner's emergence at the Crystal Scar, etc.). A book from her perspective would be awesome.