I would ask if maybe you'd want to think about this character archetype in terms suited to Runeterra
Well, as shinto priests are said to pacify spirits in order to prevent natural disasters and ensure prosperity, a runeterra one would probably calm the hunger of void creatures, ward off undead and those from the shadow isles, deal with creatures like nocturn and fiddlesticks, and stuff like that. Maybe this person could stabilize magic.
Does it have to be female?
Yes.
Does it have to be young?
20 years old would be ideal. I think people want someone who is an attractive female miku and not an old man or old lady.
Does it even have to be Ionian?
That's debatable actually. Ionian would work best. Soraka's race would work if you want to do another divine creature. I'm against it being freljord because I'm still mad Sejuani's rework gave her too much clothes and they aren't the right culture visually, it needs to be a more refined, civilized group. Noxius, Zaun, and Piltover are right out. Mt Targon is too western although the sun/moon themes could be tweaked into something more japanese.
So yes, it'll have to be either an ionian or a divine female.
I can't go into more detail on the pertinent details of what makes a Miko
I'm pretty sure what the fans want is an anime miko which is more like a priest/shaman class but is japanese themed and may use a sword, a bow, or a naginata as a weapon. Miko Ashe would probably work well as a character skin.