From the first appearance of Yhwach, we knew that there is more to him than just being ordinary Quincy. Well, maybe not just from the first appearance, but it was pretty early in the series set Yhwach's connection with god. Chapters with him as main star had "God" in name. His abilities also support it and it was spoken to him as Quincy king, which is opposite of Soul King.
On the other hand, only thing coming from Aizen was that he was just ordinary shinigami with great talent. It's not that he looks like SK, but it's that Aizen with Hogyoka's transformation looks like SK. That's because he became transcedent and what he wanted to become is new SK. We still need to see how was Hogyoku created.. I have that crack that both Aizen and Urahara were at one point in RR and that's how they got important informations about SK and one chose to stay loyal to him (Urahara) and other wanted to kill him (Aizen). Since Aizen wanted to become new SK, he with Hogyoku become like him, I'm thinking that if Yhwach becomes next SK he would look similar to him.
