Ao Shin Theory
This is a long shot. Hear me out.
The Blood Moon Theory is a thoery that after four consecutive blood moons, the world will end. Let's place this into Runeterra- there's already three blood moon skins. Two are Ionian, maybe three depending on who Kalista allies with. She looks looks like she was Ionian.
My theory is that there will be a fourth Blood Moon skin, and then Ionia will be in grave danger, hence the arrival of Ao Shin.
It's just a theory.
EDIT: The fourth blood moon (Thresh) has arrived. We will see if I am correct.