Wonder Red? Rather than seeing him as likely for Sakurai's studio visit and him being unique, I see those as basically the only reasons he'd be a blip on the radar. But look, I'll throw him up there as a possibility just so the "nothing is predictable!!!" crowd can't swoop in and be smug if he does get revealed. Though, what with Sakurai having no aversion to showing Rosalina in 2013...I wonder why he would hold off on Wonder Red around its abysmal launch?
Basically, much like the Mario contenders, none of the D-List blonde sword-users stand far and away above the others, with Shulk probably having the greatest odds due to the sheer worldwide focus his series received in 2012, right around Sakurai's taking-fan-requests period. But in the wake of Rosalina, to see none of them chosen in lieu of a more unique D-List character would not be all that surprising.