here is my first theory:
tobi is sasori.
when sasori left the sand village, he did not infact make himself into a human puppet. he instead went and lived a happy life out in some peaceful village somewhere, where he got married and had a son who looked just like him
at the age of 15, he died of that unknown diease that killed itachi, shin and kimimaro.
driven mad with grief, sasori turned his son into a human puppet and joined the akatsuki. however he did not want to show his face, so he hid in the shadows of wherever he was, maipulating the sasori jr puppet.
when sakura and chiyo "killed" sasori, he did not want to leave the akatsuki, so he got one of his other puppets (2 of his in the 100 puppet army have sharingan) and put a mask on it so that it wouldnt be discovered that it was a puppet.
(the madara who attacked konoha with the kyuubi was actualy madara. sasori saw what he could do and decided to wait untill he was asleep, then kill him and make a puppet out of him.
it makes the most scence of them all, as my tobi is hidan theory lacks explination of how he changed his hair colour
