The debate about using canon characters or even world-building limits one creativity because these are information from the "canon", than how about if we are writing a story based on Real-Life world? I don't think the creativity limit is than lesser just because Real-Life "canon" was not copyrighted.
Character proposed a tighter creativity restriction than the world-building, because character is the central piece of one's story, they are moving immediately on every sentence of the fic, unlike the overworld which is the miniascape sandbox, how the characters going to play the sands inside are all up to one's imagination.
So it is understandable one doesn't wanted to write story of canon characters, but rather OK with canon world settings. (Even I don't write story of any canon characters. Well for me, I even altered the canon PokeUniverse greatly, where the PokeUniverse in my fic is no more resembling any canon, but my own universe specifically for my fic)
The pet peeve about fanfic I had in general is: Simplistic
Characterization is too bare bone no flesh and blood no inner depth, they just talk because they need to talk, not because they have any kinds of emotional shout-out. Story plot is too basics where it can be summarize to simply character go to this place doing something then done and go to another place. Villains are there because they need to be there to stir up some drama, not because they are an integral part of the entire story and helps the growth of the protagonist. World-building are exactly copy-paste of the canon without any further explanation shall the canon failed to explained something originally, nor add on anything further or interpret differently if canon itself was illogical. Writing itself is also simplism where it is like writer deemed reader already knew everything of the canon, so dropped all the detailed description and just write the basic information.
This is especially a problem in Pokemon journey fic category.