"What IP (=Intellectual Property, be it book, movie or comic) that doesn’t have an RPG deserves it? Why?"
Oh, hey there childhood, fancy meeting you here.
First of all, I'm aware that Mouseguard is a thing. It doesn't have otters.
I started reading the Redwall series in elementary school, and didn't really stop until sometime after I graduated from high school. I fell out of the loop with the series and universe, taking up alternate fantasy authors like Robert Jordan, George R.R. Martin, and David Eddings. Reading through their works. But while just about any RPG setting can be converted to accommodate human characters in fantasy settings (And the Wheel of Time already got a D&D 3.0 sourcebook), the only anthropomorphic animal setting I've heard about reliably is Mouseguard, which currently sits at around $100- used- on Amazon.
And it doesn't have otters.
The Redwall universe has such a unique and interesting setting, with characters juxtaposed from all types of fantasy tropes. And because of the anachronistic style of product release for the series, there is a varied and plentiful history to work from, spanning generations (which, in animal years, might just mean that it encompasses like two decades). Regardless, the loving way that Jacques tells his stories, infusing them with mystery, danger, and camaraderie, really strikes home with me as a GM, and the solid cultures created for each segment of the animal kingdom means that I don't have to guess as to how I'm going to run NPCs, because I have a series of manuals to how each species acts, where they live, and what their diet consists of.
Honestly, I'm surprised someone hasn't homebrewed this already.