THANK YOU XD
Yes, Ichigo will have to fight the Soul King and yes, Aizen HAS to return, its pretty obvious.
I'm almost certain the Soul King will be the final villain. Yhwach as a character just isn't that strong in my eyes, I feel as though that his powers are so ridiculous, only Quincy can defeat him, which would be Uryu + his father(with some help from Ichigo perhaps? Or Ichigo is fighting Yhwach and while getting stomped gets interrupted by Uryu and his father), and it would be fitting too, they would avenge Katagiri and put an end to the Quincy King.
Not sure if you've heard of the manga Naruto, but they introduced a character named "Kaguya Otsutsuki" who was the progenitor of chakra, many people didn't think she would be the actual villain, so why not the Soul King? On top of that, the Soul King looks extremely powerful, unimaginably powerful, literally. What better way to end the manga on a high note than to pit the main character and his buddies (Renji + Rukia + etc...) against the King of shinigami? Also, seeing Shunsui struggling against Sternritter 'N' its pretty clear he will need help from Ukitake to take on Haschwald.
(btw im sorry my post is so messy and out of order, im really tired

_@
I also can see Shunsui + Ukitake releasing Aizen and Aizen helping Ichigo + Friends find out the weakness of the Soul King and Yhwach, then immediately trying to pull a fast one, only to get stopped by Urahara and Grimmjow.