Oh, I don't think this has ever been brought up much.
What if like the Star Fox characters, Sonic, Tails, and Shadow each had unique victory quotes for eachother? For Tails Vs. Shadow, it would be funny to have "Who's your daddy, Shadow!" from Team Sonic Racing in this game too (Though I know that line's kind of controversial since some see that as out of character for Tails heh).
Sonic and Tails would have a more friendly rivalry win quotes against eachother I imagine similar to Fox and Falco.
I still can't believe Tails, out of all people would say that.
I like how Tails from Forces was like "Hello, sir it's finally nice to meet you." to in Team Sonic Racing "Your daughter also calls me daddy(lol)."
Still, I can't imagine Tails saying that in Ultimate if he gets in. It'll be a bit friendlier.
But yeah, unique victory lines between the three of them would be neat.
Like:
Sonic, when he wins against Shadow: "Looks like it's game over for you again Shadow!" (Reference to Sonic Adventure 2)
Shadow, when he wins against Sonic: "Like I said to you before, I AM the Ultimate Lifeform." (Another reference to Sonic Adventure 2)
Sonic, when he wins against Tails: "Not bad Tails! Looks like you've been stronger!"
Tails, when he wins against Sonic: "Looks like I finally succeeded you Sonic! No hard feelings."
Shadow, when he wins against Tails: "Not bad considering your the brother of that faker."
Tails, when he wins against Shadow: "Um...... no hard feelings right Shadow?"