Re: [Theory] You do NOT need Amateratsu and Tsukoyomi to use Susanoo
It is true. To activate Susano'o, its not necessary you must have amaterasu and Tsukuyomi. The conditions are that both the MS should awaken a new power in each of them. That is the condition to activate Susano'o.
In the series, we only encountered two Uchihas until "Mystery of Tobi" chapter.
One of the Uchiha (Itachi) had MS and he showed his MS powers which were Tsukuyomi and Amaterasu. Other than that he was a specialist in using Genjutsu. Those two powers i.e. Amaterasu and Tsukuyomi, led him to awaken Susano'o.
On the other side, the another Uchiha (Sasuke) got the MS after learning about his brother's painful past. Later, he forced his MS to release "amaterasu" while saving team Taka from Hachibi. Later on, he forced his other MS to absorb/extinguish/vanish/manipulate the amaterasu flames to save Karin.
Sasuke's third power, Suano'o, awakened paritally (the Skeleton Susano'o) while fighting Raikage, Gaara and other shinobis and also when he learned about Danzo's location at the Five Kage summit. And after Danzo started mocking Sasuke about Itachi being Konoha's traitor, Sasuke released the complete (not the perfect) Susano'o.
Since the whole story was revolving around these two Uchihas, most of the people considered Amaterasu and Tsukuyomi as the required power for activating Susano'o. But that is completely false theory.
I guess the power of the MS depends upon either the circumstances laid in front of the user or the specialty in which the user is.
For example, Genjutsu user, Itachi => Tsukuyomi (specialty)
The Teleporter having most powerful Genjutsu, Shisui => KotoAmatsukami (speciality)
Sasuke => Power of extinguishing Amaterasu (circumstances faced by him)
About Amaterasu, I think that power of MS must be a hierarchical power from the descendants passed down in a Uchiha family. Otherwise, sasuke and Itachi could have got different powers. The common power among those two is Amaterasu.
And if there exist any blood relationship between Madara and Sasuke, then one of Madara's MS powers is Amaterasu.
The other power, as I said, depends upon the circumstances face by the users or the specialty of the user.
And since we know that with the overuse of the MS, the Sharingan loses its light, Itachi's vision was flickering and was about to lose its light. Then Sasuke lost his vision because of over using the MS to its extent. Same goes for Madara.
Obito has MS but his vision is still intact. Also he doesn't have the other eye.
And from this, one thing is common, that to go blind, one must have both the eyes and have acquired the third power i.e. Susano'o.
So before going blind, Madara had the Susano'o.
So the question left is what is the second power of madara's MS?
Is it awakened through his specialty or any circumstances?
The answer is simple. Through "circumstances", because the two brothers, Izuna and Madara were powerful among the Uchihas. Madara always fought Hashirama. There is something still yet to reveal "What made Madara to fight Hashirama?" And it is related to Izuna. Madara loved his brother. Also it is yet to reveal "When and How Izuna died?" and "When Izuna gave his eyes to Madara?" And answer could be Izuna gave his eyes to Madara while battling against the Senjuus in order to protect the Uchihas. That made him to go back to the "other side".
But Madara is yet to show his other MS power. So we should wait to see it but its definitely not another Tsukuyomi.