Agreed. I have always felt Tails was a strong contender to rep Sega in Smash. I do not doubt that Shadow edges (pun intended) out Tails on popularity and that Shadow could likely make the jump from AT to echo, but aside from Luigi (and now Ken), Tails is the other historically important "player 2."
I see Tails getting in before any 3rd party villain, such as Sephiroth, Eggman, and Wily for example.
Of course, Sega owns Atlus, so speculating on a promotional Shin Megami Tensei V character would make sense. However, not much/enough is known about the game, let alone it's release, to warrant major consideration. Assuming Sakurai is not a massive Streets of Rage or Golden Axe fan, then I think it's safe to assume Tails is actually a respectable frontrunner in DLC speculation this cycle.
Assuming we get all third parties, Geno, BK, Lloyd, Monster Hunter, and Tails would be a pretty bright timeline. Although, I do think Rayman, Ryu Hayabusa, and Doomguy are potential wildcards.