Hello! While I grant you that many champions are... um... strongly inspired by real world things a lot of the time, I would ask if maybe you'd want to think about this character archetype in terms suited to Runeterra (or Ionia specifically) instead of real world things like the Shinto religion.
For example - you mentioned that Ionia probably has temples - and it absolutely does! But I'd caution against assuming that those temples are carbon copies of Japanese places of worship - instead, what would the guardian of an Ionian temple be like? What would their ceremonial clothing signify based on Ionian ideals, rather than anything specifically real-world (or real world myths and stories). What forces and challenges have helped form their priorities.
Usual disclaimer that I am not a Rioter who works on League directly - so I can't help much - but I can recommend taking a closer look at what you find appealing about the archetype and looking at it through a League lens. Is it the dedication to quiet service that appeals? Magic use through the written word? The costume? (Problematic).
I'm no expert on Japanese culture, so I can't go into more detail on the pertinent details of what makes a Miko and what elements of the archetype would inspire a League character, and what other inspiration points it might work well with.
For examples -
- Does it have to be female?
- Would the guardians in Ionian temples be as likely to be male as female (or neither).
- Does it have to be young?
- Could it be something that very old Ionians dedicate their final years to guarding - because the evils held captive within corrupt those nearby, shortening their remaining days.
- Does it even have to be Ionian?
- What would the guardian of a Demacian official temple look like? How about the guardian of a run-down and near-forgotten Zaun temple? Do they even have temples?
I know it may seem unfair for me to ask you these questions, but these are the sorts of questions I'd be asking myself if I were trying to come up with a champ concept.
and 
defected and 
is rampaging in Piltover) It should be a long time (at least a year or two) before we revisit them.