'The first time he (or she) makes their fighting game debut'
We already knew that Mega Man and Asura were shot down as the fifth secret character in Ultra Street Fighter IV, but now we have more information to clarify the situation a bit -- this new character has never been in a fighting game before...ever. Capcom's David Hinds also confirms that the character is "very much related to Street Fighter." While this rules out Batsu (or any perfect-fit Rivals Schools characters), on the other hand, I'm excited to see who exactly this mystery fighter is.
I think given the heavy relation of Final Fight to the Street Fighter series as a whole, we may see a beat 'em up character make their fighting debut (since the franchise doesn't technically consist of "fighting games"), but we'll know soon enough as Ultra SFIV gears up for an early 2014 release. Personally, I have my suspicions about Carlos, since his companion Maki was technically in Capcom vs. SNK 2, but Final Fight 3's Lucia and Dean are viable options as well. Or maybe Goutetsu?
In other news, Capcom revealed that they've obtained over "18,000" responses from North America and Europe, and "12,000" responses from Japan in terms of what fans want out of the Ultra update, so the good news is they're listening.
Capcom has confirmed that they're not nerfing top tier characters for the Ultra update, but rather re-balancing lower ones to make them viable (Cody in particular is confirmed for an overhaul). So while your favorite character may not be drastically altered, be prepared to learn a ton of new match-ups beyond the five new Ultra characters.