I believe you are talking about
http://naruto.wikia.com/wiki/Kagami_Uchiha. He is a slim possiblity, but like you said. I doubt they will use someone who was in 1 chapter as one of the main antagonists. Although he is the only one that we know of that could still be alive. Albeit, he would be extremely hold. Same age as Sarutobi since they were in the same team, give or take a few years.
Judging from this picture,
http://3.bp.blogspot.com/_FLiLzPQTZ1...f_his_mask.JPG, he could very well be that age too.
I totally forgot it was Tobi who explained the Izuna thing. Then for all we know it could be Izuna. And I agree, I hope they don't introduce some random person. What was the point of the mask if its someone we don't know, right?