Wolf is a rival much like MetaKnight is to Kirby. Let's look at some facts here. We are missing a major part of this and that is the literary concepts. What I mean, all video games have a story line. Star Fox has a series of stories that shows us the relationships between characters such as Fox, Wolf, Falco and Krystal. The relationship between Fox and Falco at the beginning of the story and the games, was very strong, hence his debut in Melee. However, over time, that relationship has petered out slowly. This could be to poor planning on the writers and concepts mappers of the game, but that is the way it is.
Is anyone really that surprised that Fox falls in love with Krystal? All heroes in nearly every story written in history fall in love. It's nothing unusual. Thus, a relationship between Fox and Krystal has grown and strengthened.
Wolf and Fox are rivals. In Assault we see a deeper connection between Fox and Wolf when Wolf aids Fox for the first time ever. What does this mean? It means that the relationship between Wolf and Fox is evolving. If I am not mistakened with the story line of Star Fox Assault, Wolf helps Fox because he knows that Fox is the only person that can defeat the bad guys, and Wolf knows that he couldn't defeat them alone, hence, he joins Fox to beat them. See his moves are self interested like many villains in any story line. Wolf, to ensure his own survival against a new threat, helps Star Fox only to keep his onw tail from the grave. Along the way, he admits a little respect towards his rival, nothing too deep there.
I dont see Wolf acting in a good guy manner at all. If we base the inclusion of characters off relationships alone, Fox Krystal and WOLF will all be there and Falco will sit this one out. While Falco's relationship is shaky with Fox and this point, Wolf's relationship with Fox is evolving and growing deeper.
If we wish to take this to a gaming perspective only, Wolf still has just as much chance as Falco. Now, I hate patterns and theories, but if we look at the changes from Melee to Brawl in terms of characters, what do we see? Obviously we see an increase in rivals and bad guys and antagonists with Bowser, MetaKnight, King Dedede, and Wario. Since we have no evidence at this point of clones or characters that were clones returning, we can say that Falco's chances have lowered and Wolf's chances have increased making them equal--50/50.
In my opinion the only person that will make it in for sure, outside of Fox haha, is Krystal because she has the advantage of being in Dinosaur Planet and can have the bow-staff and magic qualities to maker her very unique not only from the the Star Fox reps, but also the rest of the female characters in Brawl. This leaves a battle between Wolf and Falco, each having a 50-50 chance.