With each Sharingan user gone the rest of the users get the sharingan stronger between the living ones...sounds like 'The One' to me (Jet Li movie)..
pretty weird idea...but i doubt its the right one.

Unlocking the Mangekyou Sharingan under extreme circumstances hmmm...this whole sharingan business to me sounds as that when put under extreme circumstances it just gets stronger..
i could be that itachi killed Shisui..obtains the sharingan we see when itachi looks at his father...then after he kills the clan he gets the even stronger mangekyou...or not.

coz sasuke developed his sharingan when he really needed it when fighting haku with naruto...but he only has one pupil in each eye...but has two in each eye as of now..cant remember how he gets the second pupils...might look up later..
i think sasuke will need three pupils in each eye before he reaches the mangekyou level. so he might get that later on and not now? under the circumstances that he protecting someone maybe..
dunno have to see what happens i s'pose..