The sprites of the mentioned characters could be replaced by some of these that I found on DeviantArt, given the due credits to the creators.
The problem is, is it possible?
I did something similar a while ago, and I managed to get the sprites to appear in the PokeMMO but they got bigger than usual. And recently I went to try to recreate the mod once again and it didn't appear in the game anymore, as if support for modifying overworld character sprites had been removed.
So, I enlist the help of someone talented enough to solve the problem, so visually unpleasant that are these gba-style characters in Hoenn.