I think Elise being Noxian is actually Riot confirmed. I always thought it would make more sense for her to recruit from bumpkin towns in the middle of nowhere, because anyone from a major city who spectates League matches knows she turns into a giant freaking spider. (Although I guess that doesn't matter anymore.)
Aside from that...
FANON WARNING FANON WARNING FANON WARNING
Just have to put that there just in case people think I'm dealing with canon Lore stuff, because I'm mostly bullshitting here.
: I like the idea that Mordekaiser is what caused the undead breakout. His old lore had him mostly hanging out in areas of plague and disease, and he has shown his ability to turn the dying into ghosts serving him in game. Maybe he's always been stuck in that suit of armor, lying in the castle basement, waiting.
: Yorick's family is all gravediggers, and he's a "disappointment" for not meeting his quota. Maybe he couldn't, considering he was around at the time of the undead rising, and is bound to fix it before he can rest, which is what is being referred to in his Judgement?
: He's into death. Might as well go where the death is. I also like the idea that he can talk people into joining with the undead, like a preacher, but for ghosts.
: Thresh's lore fits pretty neatly into the whole "Kingdom falling apart" narrative. He's a sadistic, evil jailer in life, and when the prisoners (political prisoners maybe? People who opposed the "King"?) get out and kill him, he's reborn with a similar role. Entrap the souls of those who could be threats to the King's undead kingdom, like Senna.
: Hecarim could be a knight of the old kingdom. The altar on TT tells him "Mordekaiser is not to be trusted", so perhaps he tried to stop the undead rising, and when he was killed, his spirit was brainwashed and fused with his steed in life, creating the centaur we know now. (Hecarim is an anagram of "Chimera", a creature created of multiple beings)
: Not a single clue. Maybe if there's a King, she's the Queen? Princess? Duchess?