Going back to the Tails discussion from a while back, I think he has one huge advantage over Knuckles and Shadow - he's not just a video game icon, he's a cultural icon.
and
maybe are characters I feel like you don't have to really know games in order to recognize. They're so iconic I'm sure even my grandma would say she at least recognizes most of them, even if not by name. And I think Tails (and Dr.Eggman, admittedly) are some of the last characters with that sort of recognizability not yet in the game. Yeah he's mostly known for being the sidekick of Sonic the Hedgehig, but at the same time,
he's the sidekick of Sonic the Freaking Hedgehog. I'm not kidding when I equate him to Luigi. [/I]
Oh and yeah, I didn't include Link on purpose by the way, because I honestly had no idea who he was before I played my first LoZ game. Every gamer knows him, but I probably couldn't say that about my previously mentioned Grandma.
And yeah, I know the Ballot is the primary method of choosing new fighters, but it's definitely not the only method. Simon was no where near as requested as K.Rool and Ridley, but he still made it in by his own merits. I could easily see the same for Tails. I'm not even a Sonic fan but I still see Tails as an incredibly strong contender as an original newcomer. It astonishes me how much people are sleaping on him because he's a minor background character in an age where Alfonzo McLeave-it-to-me is a thing.