It would be cool if it was Jason Todd, but Red X refers to Dick Grayson as "kid", and Jason is younger than Dick Grayson.
However he could just be toying with him.
Although he did have a cameo on Beast Boy's theory on who was Red X.
I think they never revealed it because they didn't have any good reveals. People who aren't into comics watching the show wouldn't get Jason Todd, and there weren't many other characters we knew that it would be fun to have him be revealed as.
if they didn't reveal maybe it was no one. they just had thought of no one for the job and made a whole new character, the whole mistery thing was only for thw show, not really meant to be a great mistery, otherwise it would have been answered or more obvious to answer