So the often stated theory that Shisui is Tobi is wrong?
It's actually that Tobi is Shisui?
At first, that seems like a stupid or useless distinction, but it isn't if you think about it.
It would allow Shisui to show up into the Uchiha clan relatively invisible and then slowly grow through training.
Now, people have already stated that Shisui is Tobi and then get dashed because he was around Itachi's age. They say, "Well, Tobi was around when Itachi was just a wee little kid" destroying their point. But if you approach it form the opposite and assume Shisui was NOT the original, then, yes, we can still do the Tobi is Shisui, but approach it from the opposite angle.
That could work.
However, it's still likely that Madara is Tobi and Tobi is Madara...somehow.