Wait... Can you explain the magic coincidence thingy a little more? Does it mean that coincidences only happen because magic users visualize them? I'd be happy to give you a plot idea, but my imagination bank is very low on funds right now. 

I'm not sure I fully understand this, but here's a plot. Feel free to make changes to it!

So there is a girl or boy who is living a perfectly normal life. Then, one day, they are thinking particularly strongly about an object or something, and they see a slight hazy image of the object. They think it's a hallucination. Then, it happens again a few days later, and they can actually see the full image, but when they try to touch it, it vanishes in a puff of mist-poof! The third time it happens, it is a solid, whole object. They are horrified and think they are going crazy. Then it turns out that a wizard in an alternate reality is testing their psychic powers because he needs them to go on a quest for something... that's all I can think of, and it might not be what you were thinking of, your idea wasn't that clear. 

Oooh! I just got a really good idea!!! Okay. So, how about:

A girl keeps seeing this other girl (who's a magic user) at the most random places. They seem to run into each other everywhere, the grocery store, her brother's soccer game, etc. (coincidence? I don't think so!) So, what happens is that the magic user girl is mysteriously connected to the girl without magic, and can't do anything with her magic or else her powers will be given to the non magic girl!