I think if MB is gonna give us Shizuka and Asami threesome it will be a snowball from sleeping with Asami.
Just my wild thought how it might play out, we end up sleeping with Asami first and she falls asleep in our arms, Shizuka comes to check on us and see this and gets undressed and joins the snuggling, next morning Asami wakes up and sees Shizuka there and begins to freak out a bit and wakes the rest up, small conversation nothing happened with Shizuka and then Shizuka wants to talk alone with her mother. Shizuka then does her thing of Jedi Mind Tricks and convinces Asami it is not wrong to play with MC together, 2 days later they surprise MC.
Crack pot theory sure, but we have been shown Shizuka knows how to get her way in the house and has been getting better at convincing others to her ideas, we also know Shizuka has no issue with the incest thing around MC because of who he is, Asami is accepting of it if not as openly as Shizuka.