I really like the idea of Metal Sonic being Sonic's echo, but I think given Shadow has already featured as an assist, Shadow is the likelier candidate. Plus, Smash isn't just positioning this as a "heroes vs villains" idea; the title of the K Rool reveal is 'The Rivals', so Shadow featuring as Sonic's arch-rival makes sense. Personally I'd like to see Metal Sonic or Robotnik join; they might feature as bosses. Metal Sonic could easily be another assist/boss.
I love Azuren's Giovanni idea, that'd be really cool. Other interesting echoes would be Medusa for Palutena, and Black Bull for Captain Falcon. In fact, I'd really like to see both of them in the game. Akuma as an echo for Ryu would be good, but I think Ken is likelier.
Personally I think Skull Kid is a likely addition - we've not seen his assist trophy and they've added the Moon as an assist, so Skull Kid might have been promoted from assist to fighter. He'd be a rival/villain for Young Link, who should have his sword and shield from Majora's Mask as a costume option.
The thing is we were told not to expect many newcomers and I don't know if echoes are being included as 'full' newcomers, or newcomers just count as characters with new movesets. We've had Inkling, Simon Belmont, Ridley and K Rool as complete newcomers; Daisy, Richter, Chrom, and Dark Samus as 'new' echoes (Lucina, Dark Pit as 'old' echoes). We're up to 67 unique characters; 73 including echoes; 81 including the koopalings and Alph.