I was reading the other topic on the secrets on the sharingan and that reminded me of the sharingan's true secret.
I take this from chapter 225.
(off topic: omg i got teh right chapterxors just by guessing teh number. I am teh c00lxors)
Herm, yea anyway
Chapter 225 is a flashback chapter to when Itachi left after having killed the entire clan, leaving Sasuke alive.
He tells Sasuke that "At the main temple of the nanako shrine... on the far right side, under the 7th tatami mat is the clan's secret meeting place. There you will find what purpose the doujutsu of the Uchiha clan oroginally served... The real secret is hidden there."
So Sasuke heads there and finds the mat and finds the secret meeting place. He then says: "I see, so that's how it is", in front of a stone tablet.
Anyways this is just a conjecture topic as to what the real secret could be, what the original purpose of the sharingan was.
I really can't think of anything right now, maybe post something laters.