Well i have two theories about this game plot so its like this:
In the end of third Fantasy Maiden Wars, Mima was talking with someone unknown and i think its Byakuren. I don't know why but both of them are Magicians. And also since the start of the first FMW(Scarlet), someone has been talking with Marisa and it might be Byakuren.
Then the other theory, Yuuka was also talking with someone and it might be Shinki, because maybe Shinki orders Yuuka to keep an eye on Alice, since in FMW(You) Yuuka appeared at Alice's house and again at the credits of FMW(Ei), and maybe what if Shiro(Kedama of Marisa) was given the power of Shinki by Yuuka to posses Utsuho?
So yeah the thing about going to Makai to rescue Byakuren might be true.
And the SWR might be one chapter like IAMP.
Any other guesses for characters appearing?
I hope that Kogasa and Mamizou appear since they are the only 2 left of the Palanquin Ship crew missing...
And for last Koishi might be a boss too, since we couldn't fight her and in the credits she looks very different...