As of right now, I'm almost certain K.Rool will happen. The fact that we have a kostume of him that looks like it was ripped from an actual model? If he is not in this game, I will be VERY surprised.
And judging by the whole "fan service" comment, I predict we will see the following kharacters joining the battle VERY soon.
-) King K.Rool
-) Inkling (Was feeling kinda "meh" on this one, but when you think about how large the Splatoon community is now...)
-) Wolf (Sometime after Star Fox Zero comes out. I wish that game continued from the original story...)
-) Isaac (Need I say more? Every other person I see is either asking for K.Rool or Isaac.)
And POSSIBLY...
-) Bandanna Dee (Not quite one of Sakurai's own children, but he'd be great for four-player Kirby matches. <3 He's been in a lot of Kirby games lately, and even though most of them are gimmicky and somewhat boring, it would make sense to put him in...)
And KINDA NOT POSSIBLY...
-) Banjo-Kazooie (I'm REALLY unsure of this one, but... that bear Mii kostume felt a little random alongside everyone else...)
-) Snake (There is quite some demand for him, but due to the recent Konami situation, I doubt it. Seeing him more "classic-ified" like Pac-Man, Sonic, Mega-Man, and Ryu? That'd be pretty sweet.)
-) Shantae (Not as popular as most people think. The only way I'd see her getting in is if WayForward keeps begging Sakurai like they are now... -_-)
-) Shovel Knight (Unless this new, exclusive Nintendo content for him features a Battleship Halberd level, and a fight against Meta Knight. Then Sakurai would be pleased.)
-) Rayman (Should have been DLC right after Mewtwo and before Lucas, but you never know!)