@Aaronrules380:
I think they usually don't disappear until a new character is going to be shown soon. At the very least, I don't think new characters have been shown until the last banner was taken down
Wrong.
Back in the past year there was a thread on Smashboards trying to follow patterns such as this one, and there were many cases of a character banner disappearing causing many people to go nuts over a reveal soon. As someone who also tried to find hints and stuff for the sake of hype, I remember clearly this rule not actually existing.
Not only was it inconsistent how long it took after a banner went down for a character to emerge, there were also cases of the character banners still being there right until the reveal of a completely different character. From the top of my head I feel like Ike's banner was there up until E3, and there was also the situation of Diddy Kong appearing while Little Mac's was still up, the banner remaining up for Zelda right after Rosalina, and then for King Dedede right after Zelda.
From looking it up apparently the same was for Sonic.
So nope, the banner means nothing. As for the case of newcomers, usually we get much more concrete evidence that one is happening anyway, namely Sakurai telling us to look out for something.