I think that there will be a jump on the time of some years after the final scene (maybe an explosion, a kiss or anything like that), we will see Ike and Yoshida as a couple, Satou will become a Flame Haze, since he is the son of some important people I guess his Potential of Existence is enough, the funny part is that I spect Valac to be his contractor, since she is looking for a new vessel after the Yurī Chvojka's death, maybe become the Margery's partner as Chiara and Karl.
Ogata and Tanaka as a couple. The Yuji's parents with the newborn baby. And I guess that either of Shana traveling around the world with Yuji, or he will be dead and just a far memory that only the Flame Hazes know. But well, that last part can change dramastically, and I think that all the theories are complete wrong as the author is going to make something unspected, and as soon as someone thinks about it, he changes it
