I smell a parallel coming....
Seriously, Konoha love love triangles have some serious direct and indirect consequences. So I'm guessing Mito enters in the equation somewhere. Either she loves Madara and he ignores her until it's too late or she scorns his love for Hashirama's. (But ignoring her until it's too late, undervaluing her worth until he sees he needs it (because she turns out to control the 9-tails) would be such an Uchiha thing to do!) Anyway...however it turns out, I smell another parallel coming!
For a peace-loving village, they sure hold some devastating grudges, you know?!
As for the symbol.... How can you think it's not tied to Sakura? Kishimoto could have put anything on Hashirama's forhead. A box with a check in it. A steaming bowl of noodles. And if he did, then we could expect a story bump for Teuchi and Ichirakus.
What I'm saying is the man loves clues, foreshadows and parallels. He could have put anything...but he put a sign that
IS Sakura's symbol, minus one usual detail.
If this turns out to be some sort of slug contract, then the tie in might be that once Sakura inherits it, the symbol becoms complete for her. The circle gets the dot in it when it's activated.
Look at Tsunade when her regeneration jutsu is activated fully — diamond with swirls coming off her forehead and down around her cheeks.
So why couldn't Sakura inherit some justsu that functions in the same way, with her clan seal involved? A circumpunct circle/dot on the forehead, with swirls coming off her forehead and down her cheeks.
Makes sense to me. And Kishimoto would have a lot less explaining to do if Sakura got a sudden powerup, after the audience already knows where that particular power-up has come from.
Oh yeah...isn't it the picture where Sakura, Naruto and Sasuke are leaning against the wall and their clan symbols are above them? I think that her clan symbol is dot-within-a-circle there.
Oh and great, great post about the circumpunct, CynMichi! Really interesting!