In order for Knuckles to be in Brawl, Sega needs to have a second character (obviously). This may not be too likely, but I do think it's possible. After all, Nintendo and Sega are seemingly pretty close at the moment. So, let's say Sega is allowed a 2nd character. I think there are really only two options: Tails or Knuckles. A lot of people say Tails is the hands down choice, but I don't see why.
Knuckles could potentially act as a much needed villain for SEE
Knuckles has a little better moveset potential imo, with the punching, gliding, climbing, and digging (but this probably doesn't matter too much because Tails also could have a pretty good moveset)
Knuckles is more of a fan favorite.
The only real thing Tails has on Knuckles is that he existed first (in Sonic 2 as opposed to Sonic 3).
However, both characters just seem like perfect candidates for AT's. So I doubt either will be in. But here's hoping for Knuckles.
Knuckles could potentially act as a much needed villain for SEE
Knuckles has a little better moveset potential imo, with the punching, gliding, climbing, and digging (but this probably doesn't matter too much because Tails also could have a pretty good moveset)
Knuckles is more of a fan favorite.
The only real thing Tails has on Knuckles is that he existed first (in Sonic 2 as opposed to Sonic 3).
However, both characters just seem like perfect candidates for AT's. So I doubt either will be in. But here's hoping for Knuckles.