Now, that might sound ridiculously strange, but I believe that out of Bandai-Namco's candidates it would be Agumon who is the likeliest candidate mostly because of just how much bigger he is than basically anybody else Bamco has. Like, even bigger than Tekken. His sales records franchise-wise don't lie. And the guy actually had quite a bit more Smash support than I thought. Not to mention he represents something new, as well as that whole "Pikachu rival" angle that is basically a pretty big deal. And if Rumble Arena is meant for a "no-competition clause", well it's a good thing they came out ages ago and haven't seen a rerelease.
(Plus, Pokémon is also way more known for the anime and TGC among non-fans, though sales of the games are second only to the merchandise. And Pokémon is first party.)
KOS-MOS, on the other hand, I had hardly seen get requested for Smash Bros. She has a fanbase, and it's not nonexistant, but going around the block her name doesn't come up very often. Xenosaga's impact isn't very high when you put it next to Digimon in general, either, and while I hate to pull out the dead series card, from my understanding her home series hasn't seen an entry since 2006. Meaning she mostly survives on crossovers, kinda like Morrigan. So, besides the "she's in everything" angle, KOS-MOS doesn't have a pretty big claim to it to me.
I don't even care about Agumon personally, the only Bamco rep I do is Lloyd. But the way I see it, Agumon is the likeliest. Is he a shoo-in? No. Will he be the first? Not necessarily, but I believe he will unless Sakurai has something to say on the matter.