Queens vengeance
We know the uldren is alive. He is currently within the house of kings trying to get close to their kell.
We can assume Mara Sov is alive through what the voice actress said about it a while back.
Taken Queen
Yes I can almost promise Savathun will show up in the game and personally I can't wait. However, at the end of the taken king, Eris is talking to Queen Mara Sov. Also, at the beginning of the taken king in the cutscene, Mara is talking to Eris.
English
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This theory could be accurate, but more than likely most of it will be wrong