First off let me note that I DO NOT remember every detail of the manga, so if im wrong somewhere please feel free to correct me. Down to bussiness. The manga has really taken a twist. we have just seen madara uchiha revived by the Impure world ressurection. we all know that you must be dead for this justu to revive you. based on common sense, this proves without a doubt that the masked uchiha(ill just call him tobi) is Not Madara.
Some theories:

Obito: No, get off of his nuts please obito is dead. He has no sharingan left(one crushed, one taken by Kakashi.) This uchiha had two sharingan(before he used inzangi).

Inzuna: if i didnt spell it right sorry but its madara uchiha brother. while it could be, i highly doubt it. lets face it too many stories were told of madara taking his brother eyes and his EMS just make it even more proof. i believe he is dead.

Madara: No, why would madara be afriad of himself? many more conflicting factors but im not getting into this.

Sasuke/Itachi's dad?: hmmm could be, but itachi slew him with his own hands. i dont think itachi would have left him alive.

My theory: simple. A uchiha who has yet been introduced. Until futher information is revealed i believe this is a uchiha who might have been envious to madara. he is obviously powerful(even though he hasnt shown any impressive combat experience). What gets me is why has he been alive so long?
Also why was he able to perform inzangi? Maybe these two (his age and the use of inzangi)are related? Uchiha and Senju blood?Uzumaki's have long lives and a strong life force so i assume so does the senju. Last but not lease. This is a long shot but im gonna just say it. although we never seen it he HAS MANGEKYO SHARINGAN. If you ever notice when he uses the space time technique, it always comes from his eye. this has to be his mangekyo technique. I could go on and on but i would like to hear others. what do you think?