I think Arle is very likely to get in, and would fit being character 5 in the Fighter Pass, but something about it feels off. Like, if it can't be Crash and it can't be Ryu, I really think it's going to be Lloyd, or maybe Doom Slayer or Reimu. Arle is likely post-pass, but... I unno.
KOS-MOS, on the other hand, out of the two has a much more credible shot. Popularity goes a long way, even if I think SEGA having a character and Namco having none won't matter for very long. Still, when you put her next to Agumon you really can't say much about her, I don't see her coming before Agumon, even if we assume Lloyd and Heihachi are just Mii Costumes waiting to happen. Otherwise, I would say she has to put up with them, and between Lloyd's Smash popularity and Heihachi being the face the former biggest Fighting game franchise, things don't look good for her being very soon on the Bamco tier list.
Kiryu is another character with a solid shot. Don't have much to say on him, but depending on how long DLC goes on for, we could see both him and Arle show up. I imagine that Arle, who has the longer history and is actually the fan favorite of the two Smash-wise, would happen first, and Kiryu sometime later. If we only get one, I must admit Kiryu has a good claim to being the one, but I still believe Arle edges him out a fair bit.
I have no faith in DLC Mii Costumes simply for business reasons, but if even Sakurai doesn't care about details like that, time and again he has shown preference for new series over current series reps. Tails doesn't attract any more of an audience than Sonic does, and that's not really a good thing when they have to pay for the rights to use him. I feel like if they want to do a second Sonic character, Eggman (who has way more potential for uniqueness and is as big of a name as Sonic) would be barely more likely, even before taking into account he's not a DLC Mii Costume.