My personal opinion about more reps from third party series is that if the character has enough to offer on their own, I don't see why it's worth denying them just because they aren't the face of their franchise.
Like in this case, Shadow. He isn't the protagonist by any means (though debatable in a few occasions, which can be done for others like Zero from MMX or Ken from SF) but he has a lot of unique abilities that he can bring to Smash. Should he (or others) be disregarded just because the third party series already has their one rep? Personally I don't think so.
The likelihood isn't high in the first place I'll admit due to the nature of third parties, but there are some really worthwhile characters that get swept under the rug because of this.
Also, I'd probably prefer to see a crossover platformer or racing game between Mario & Sonic than a fighting game tbh. Or Super Smash Kart.