I feel that Diablo's master plan is to even absorb all of the Archangels as well. Now, with Diablo taking sole control of the Blacksoul Stone makes me think that it was his testing grounds to his bigger plan. If he can control all of the Lords of Hell, then why not also the angels? So instead of us just facing Tathamet, we face the all mighty Anu with all his strength of Good and Evil.Quote from Bleu42
I think ever since Diablo failed in D2, and Baal failed to corrupt the worldstone, Diablo has finally decided to go for his master plan; Combine all the lords of hell, and re-create the 7 headed dragon Tathamet.
According to the lore, There was Tathamet and Anu who fought each other in the beginning of creation. Both died, and both of their bodies resting places created the burning hells and the high heavens. Anu's spine is what creates angels, and Tathamet's eyeball (Can't remember, I think it's his eye) is what creates demons.
Now for what happens at the end of RoS. Some of this info is based off stuff that was datamined. Read at your own risk.
Now, a while back, there was datamining for the cinematics you receive when you play as a Crusader. They even datamined the ending. So yes, we kill Malthael. But during the cinematic, Tyreal witnessed the strength of the Nephalem. That they, in fact, have no foreseeable bounds of their power. What I think happens, based upon that, is the Angels will realize that it was a mistake to support us through everything. So a war breaks out amongst the angels. Should they kill the Nephalem or should they live? During that turmoil, the Lords of Hell will somehow corrupt someone and use them to break free once again and cause havoc. Except this time, I think they will corrupt an angel. Not any angel, an archangel. And help lead to receiving a power just as great, if not greater than the nephalem.Now, another theory I have, is that we will go back in time. Diablo regains control over a body and somehow goes back in time. He wants to change the Worldstone again. But with stronger powers. He knew that they were weak still when they were on Sanctuary back then. They had a host that their power reflected out of. And the Worldstone, I believe, prevented them from using their whole power. So with the new found power, and a stronger body, he goes back in time to change the Worldstone. Instead of corrupting it, he changes it. It instead keeps Angels out of Sanctuary and lets demons have full reign. It also weakens the nephalem and gives the power that they lost to the Lords of Hell, or just Diablo himself. He is close to reaching his goal when we are actually notified and by the time we go to save the past, it is too late and Diablo has achieved his plans. However, it was not an instant power-up. So we race to find Diablo, saving towns and countries, and even building up an army, to confront Diablo. It is like Dragon Age: Origins but in Diablo. XD That ending I would love to see, but the premise still remains. We have to stop the Lords of Hell from messing with the past.