I've always personally liked the concept of "kid gets changed into a Pokémon and it's
not Mystery Dungeon time," but there's something that you may want to consider: Maria herself.
First off, you'll probably get a few raised eyebrows over the idea that Ash and company are her longtime friends. Unless this is a canon character, the first question people are going to ask is probably going to be, "If she's a long-time friend, why wasn't she ever mentioned?" As in, it's really a lot harder to shoehorn an OC into the group because it means trying to work around the problem that Ash and company just wouldn't completely ignore or never mention someone they're close friends with. Ergo, it ends up being incredibly awkward to just have her appear with little to no prior explanation as to why she was never even mentioned.
Basically, fanfiction's based on a fan's what ifs, of course, but the writer needs to remember that canon is the least common denominator in a fan's mind. As in, you don't violate canon in part because you're a fan (and are therefore doing this out of respect) and in part because if you do, you're going to confuse the crap out of your readers. That and it's just easier to acknowledge and work
with canon than it is trying to come up with the explanations to avoid the second problem (in which the fan gets incredibly confused because a random character appeared, claiming to be one of Ash's best buddies when Ash hadn't even mentioned this girl canonically).
Second, you'll want to be wary of the concept of a
Mary Sue, or a character who suddenly appears and tries to bend reality to her will. I say this mostly because, well, one of the classic examples of a Mary Sue is the ultra-awesome character who, to the readers, appears out of nowhere and happens to be the canon characters' BFFs that just were never mentioned in canon. (There's parodies of this, even, particularly in the Harry Potter fandom, in which every Mary Sue tries to be the daughter of Severus Snape for some reason.)
Rambling aside, while the concept itself is interesting, I'd probably try to rethink Maria herself to avoid treading awkward points and the Land of Sue. Perhaps, for example, she met Ash later instead of being a long-term friend. Perhaps she doesn't know him at all.
Or, perhaps, you could consider using one of Ash's actual long-time friends as well, such as Misty, May, or Dawn. Is it necessary that she has to be an OC?
In any case, good luck, and I hope you think carefully about your characters.