All right, I've been virtually inactive here recently due to the wiki's general activity, but I thought I'd put something up for any future new users to look at, and it's certainly been far too long since I've done an essay here (The last one I posted was in the fall of 2011! Nearly 2 years ago!)
Not sure if I'm going to add more; though the first section is going to be about villains, so without further ado let's begin.
Species and your baddie[]
I've noticed that quite a lot of users here seem to have pine martens as their story's main villain; which is ironic given that Brian Jacques (may he rest in peace) only ever used three over the course of the series. That being said, we don't really have a clear idea of what creatures of this species are typically like (Since it IS known throughout the series that hares are typically obsessed with food, rats and weasels are your comic relief lackeys, foxes are arrogant healers who are sometimes fraudulent, etc); though with Ublaz being a key exception, the only other two pine martens known, Atunra and Ashleg were merely associates to more powerful rulers.
That being said; because the species doesn't really have a status quo, it also gives you freedom to be more creative with your main villain, assuming that they're a pine marten; though I would strongly advise that you not base them off of Ublaz, given that that would be ripping off the books somewhat.
Notably, the pine marten also has several subspecies; most of which seem to live in Asia. If you wanted to make your villain one of these creatures I would recommend it; given as they're not that dramatically different from a known, in-universe species and therefore would probably fit the Mossflower setting well.
Before I close, however, I really must point out: Just because your villain is a certain species doesn't necessarily entitle them to special powers. Yes, a few villains were vain enough to have symbols of power in their respective books (Ublaz, Gabool, Vilaya, etc), but said icons would only end up killing them later, and really the only villain with any sort of especially unusual skill was Ublaz with his hypnotism; but that was something he LEARNED, and not just because of his species. Don't use your villains' species as an excuse for them to have some random magical ability! Especially if it doesn't exist in the canon universe!
Be creative[]
Far too often I've seen fanfiction writers on here attempt to emulate the writing of Brian Jacques. There's nothing wrong with that; it IS fanfiction after all! However, I really must say that due to the fact that nobody here is BJ, you don't necessarily need to follow his black-and-white (mostly) moralities, or formulaic plots! In fact, writing fanfiction opens far more avenues to creativity in the world of Redwall, so simply use your own plots, plot devices, etc. It makes for a much more original story.
And those are my opinions on effectively writing Redwall fanfiction! I plan to post more in Part 2, so stay tuned.