I can hardly believe this; 21 chapters and no reviews? Quite a shame, as I've just read the first three chapters and I'm rather enjoying it.
When I read those prequels you had posted, I had been of the opinion that your Brock seemed like the anime Brock in name only, but now that I've read this much of the story proper, I can see enough of the little things that make them more similar. The same goes for Ash. Even though he's a mage now, which at first seemed like an odd match for me, his personality is recognizable and the juxtaposition doesn't feel awkward. I think you have a good balance between the original and borrowed aspects of your canon characters.
Another thing I want to praise is the familiar system. It's a very interesting way of handling an analogue to Pokeballs and the traditional trainer concept in this non-technological world. I'm also a sucker for anything resembling a music-based game interface. The fact that Lugia's summoning tune is longer and more complex than Pikachu's is also a nice touch. Speaking of Lugia, ordinarily I have a problem when characters spend too much time on dialogue that's almost entirely expository, but I think it works pretty well in this story because of how interesting your new world is.
As in the prequels, Brock's story is charming and amusing. I'm going to guess that there are more of them, and that's more than fine by me.
The writing seems fairly well polished, and I don't recall finding any grammatical errors, though I wasn't looking terribly hard and I might have missed some. There were a few things I found problematic content-wise, though:
I'm impressed with what I've read so far, and I'll try to catch up with where you are right now in a few days. For future reference, I wouldn't update quite as quickly as you have for this fic. That might be a contributing factor to the lack of reviews because long fics-in-progress are less likely to get new reviewers, unfortunately. I know I don't usually start reviewing when the author is this late in the game, but your prequels compelled me to start and I'm glad I did.