As we all pretty much know, most, if not all, Redwall novels have had the foxes/vixens in them, but there's one thing they have in common; they're portrayed as either the villains (in the case of Marlfox) or as the villain's lackeys, most skilled warrior(s), etc. I bring forth an argument that I'm pretty sure is gonna be deemed as the most stupidest, least thought-out theory you've ever read; Foxes/Vixens could just as well be the hero/heroine as any other race in the whole entire Redwall universe.
Before you say or think to yourself, "what's this idiot saying?" just hear me out! In The Outcast of Redwall, the hero was a ferret, which is a typical classic Brian Jacques villain class. He was seen as the villain until he sacrificed himself to save those who had seen him in this way, AKA the denizens of Redwall. He was afterward given the respect he deserved when he was still living, showing that even the person you expected to kill you could be the one who saves you sorry rump at some point!! Now imagine this happening to a fox or a vixen, minus the death part. They, like the other villains in the novels, are seen as "vermin" and isn't to be counted on during any fight alongside any hero. Imagine a fox coming to aid Matthias, Deyna, or other Redwall hero/heroine or a vixen healing the wounds of some poor woodland creature and even accompanying them on an important quest. What if Martin the Warrior foretold that a fox or vixen would come to the aid of Redwall in its time of need? How would the creatures of Mossflower and Redwall react to this notion? They'd probably think Martin's lost his mind or he might be talking about a mouse or a badger that looks like a fox/vixen. To that, someone would probably say that Martin wouldn't like about this kind of thing and that he wouldn't send a vermin to help them if another option was available. Still take some convincing, but everyone would have no other option but to realize that heroes are just mice, hedgehogs, badgers, etc. They can the person you least expect it to be. And when Martin says it's a fox, weasel, ferret, rat, etc. that's come to save Mossflower, you can bet your slice of Grayling a la Redwall that's what he means! No one has ever doubted what Martin the Warrior has ever said, so when a vermin ends up being your savior, just roll with it! Oops! Rambled on!! I do that a lot! Anyway, to conclude, if a ferret can end up being the hero, then why can't a fox? If all of Redwall's greatest heroes have been woodland creatures, then what's stopping a fox from going up to Redwall and saying, "I had this weird dream that I was supposed to come here to protect you from a great threat, despite the fact you all hate my guts to hell gates."? I rest my case...I think? I think I made a pretty good argument. Don't ye?