It pains me to say it, but any Digimon rep if the series is to have faithful representation. Sure Rumble Arena 2 is a Smash-like Digimon platform fighter, although to truly get a Digimon represented, you'd have to include digivolution as a mechanic. For example, say that Dorumon ends up being the representative for Digimon in Smash Bros theoretically (picked in this case because he's my favorite Digimon). Dorumon is in his Child/Rookie stage, which would be the base. Like in Rumble Arena 2, a Digimon would gradually increase their digivolution bar to digivolve into later stages. After Child/Rookie comes Adult/Champion (Dorugamon), Perfect/Ultimate (DoruGreymon), and Ultimate/Mega (Dorugoramon). That's a total of four forms for a fighter to have. Rumble Arena 2 shafted this by only giving Digimon three forms (and having bizarre digivolutions like Flamedramon becoming Imperialdramon, when something like ExVeemon or Paildramon would have made more sense). But to truly get a Digimon's family across, all four stages are a must to be faithful. Yes Baby/Fresh and Baby II/In-Training are also a thing, but in many Digimon games those forms are too weak for combat, so they can be excused (although seeing them as potential Spirits would still be nice if Digimon ever did get in).
Then there's the virtual pet mechanics, since Digimon first started as Tamagotchis designed to appeal more to males by Bandai. Of course, now it can also be debated on if starting off as a Tamagotchi counts as being a video game rep, but that's another debate. Sure you could have the Digimon character have a down special that incorporates the V-Pet mechanics, but that brings up another regard: direct control. In the Digimon games more faithful to their V-Pet origins such as Digimon World 1 & Next Order, you don't have direct control, instead they act automatically. This isn't the case for other Digimon games however, since Digimon doesn't really abide to following a certain format unlike the main series Pokemon titles. But it is something to keep in mind when pondering about the idea of a faithful Digimon rep. I highly doubt Sakurai would allow for a fighter that's completely AI-controlled though, but even besides that, all the care-taking mechanics in V-Pets would be something decently considered.
Sure it's possible to pick something like a high-level Mega such as Omnimon and call it a day by not including any of the mechanics I discussed in the paragraphs above. But that's like not incorporating Kirby's ability to copy opponents in Smash, Link's arsenal of tools, Mr. Game & Watch's references, and etc. When I think of a character that's incredibly faithful, I think of Simon Belmont since he represents his franchise in such a satisfying, well-crafted way that shows true dedication. On the opposite end, when I think of a character that could use some severe updating to be more faithful and represent their series better, I think of Samus. And as a long-time Digimon fan, I would rather have a Digimon be represented like Simon is rather than the insufficient, half-baked way Samus is.
Overall, I don't think a Digimon rep would be impossible in Smash, but to represent the franchise in a way that would satisfy fans, it would be incredibly difficult and time-consuming, with a lot of issues that would have to be addressed in order to ensure Digimon would smoothly translate into the world of Smash.
