Yea, another theory, but this one isnt whimsical and actually thought out so give it chance... And please read and consider everything I have presented here before coming to a conclusion.
Introduction: Recent evidence had me wondering if shisui=tobi, but after examining all the evidence I believe its very likely Tobi=shisui, rather, tobi was posing as the uchiha known as shisui to itachi, the mist village and the rest of the uchiha.
Exhibit A: Ao recognizes the chakra in danzous arm and eye as shishuis... (last two panels)
http://www.mangareader.net/93-22827-...apter-459.html
However, come to find out the power danzou is truely using is hashiramas after incoperating his cells for super chakra and stamina to use Izanagi and 11 SGs.
http://www.mangareader.net/93-44811-...apter-478.html
So shisui used a hashi powered body with the SG, as implied by Ao recognizing the color of the chakra.
exhibit B: Tobi is the only known user of hashis power along with the SG.
http://www.mangareader.net/naruto/545/4
Plus, Ao confirmed that it was shisuis eye power that was controlling the 4th mizukage.
http://www.mangareader.net/93-22827-...apter-459.html
And we saw it was apparently tobi who was controlling the 4th mizukage
http://www.mangareader.net/93-56935-...apter-507.html
So it had to of been tobi posing as shisui.
Exhibit C: Tobi is the only other shinobi that has developed a MS level jutsu with a mere basic SG (his t/S jutsu). And the only other MS level jutsu with a basic SG just happens to be shisuis Doryuku that controls the thoughts of others. And if tobi is the only other shinobi that has deveopled a MS level jutsu with a basic SG coupled with all other evidence I have provided,then its most likely tobi was shisui.
Motive:
And since I doubt Shisui is some random senju/uchiha hybrid, Its most likely that tobi is shisui, rather played the role to manipulate itachi and the uchiha. In order to win over itachi and play the role as his best friend all so he could force itachi to kill him and awaken the MS, thus completing step 1 in the process of reviving the power of EMS. And I assume tobi was actaully there as himself, not a zetsu clone which wouldnt of been strong enough to use doryuku. Zetsu clones are too weak to use KKGs.
Tobi probably used a zetsu clone in his fight with itachi which is why kabuto cant find shisuis body anywhere despite being so good at finding bodies.
http://www.mangareader.net/naruto/520/14
Although I could see tobi using his Doryuku mind control genjutsu to make those he comes in contact with believe hes always been there. But, somehow danzou mustve found him out and tobi offered to destroy the uchiha if itachi could join akatsuki. Still, danzou is greedy so Im sure the final stipulation was tobi having to give away his doryuku so danzou could use it to become Hokage later. Danzou did confirm they meet during the massacre so a deal between the two isnt out of the question.
http://www.mangareader.net/93-41103-...apter-474.html
Conclusion: Alot of evidence points toward tobi playing the role of shisui back in konaha as well as the mist village. With tobi bieng the only other who uses hashi power with a MS level basic SG, Ao recognizing the power as shisuis then a shinobi very similar to tobi being shown controlling the 4th mizukage. And theres the fact kabuto couldnt find his body? Why? Clearly he isnt dead or rather wasnt the real shisui, but a zetsu clone whos now nothing, but a plant.