I’m unfamiliar with Digimon’s origins. Did it really start off as a video game?
As far as I know, it started as a Tamagochi-like virtual pet LCD game, so a videogame indeed, even though it's popular mostly due to the anime.
Though I think that people are taking the "must be from a videogame" rule too much seriously. I'm not talking of Goku and Shrek, but there could be exceptions such as maybe an indie developer writes a book or something, then makes a game based on the story of the book, and the game becomes a success, while the book is a very obscure one and a flop... it would be a big waste to consider that game not viable for Smash only because an obscure book existed before.
It's just an example, maybe even an unrealistic one, but I think it makes the point clear.
EDIT: Another example, Pokèmon is mostly a game franchise, but there was a time, around the series' debut, when the games and the anime were just as important as each other. That time was when Pikachu and Jigglypuff were added to Smash.
I honestly think that as long as a game is big enough, it doesn't matter if other media exist, it can be in Smash. I think that Sora/Kingdom Hearts can totally fit Smash, even with the Disney cameos, because the game series is major enough.
On the other hand, while I'm not against Digimon, I don't think that the Digimon games are popular enough to justify Smash representation... but that's just me. I would be cool with Agumon, don't get me wrong, it's jsut that I see it more as an anime character than as a game character.