Kamen Rider Saber × Ghost ended with Kanon getting taken away by Kamiiyu & Muller. Jabel couldn't save her. Seems like there is a sequel to this. But it's not an crossover with Saber.
So what could it be? and who will be the focus of sequel?
This could most probably be a sequel focused on Alain / Kamen Rider Necrom. Because TTFC was promoting The Legend of Hero Alain with the announcement of Saber × Ghost even though Alain wasn't even in this story.
In the Saber × Ghost spin-off, Takeru gets a call from Gamma World and that's about Kanon going missing. Though it was not mentioned who was the caller, it could most probably be Alain who is worried about Kanon.
The mysterious man who is seen in the shadows in Saber × Ghost was never revealed. He was just credited as "Mysterious Man" with no details of who is playing it. It was just "????".
This could be a character related to Ghost since it could still have a continuation.
Kamen Rider Saber × Ghost messes up the Saber TV Series Timeline.
Tassel had lost his power of existence in Chapter 26.
Touma had decided to write the Sequel to Lost Memory in Chapter 28.
In the spin-off, Tassel is seen doing the narration. So it means he is still existing.
But you also hear from Mei asking Touma about what's happening with the Sequel of Lost Memory.
Now, it's difficult to align the things.
You have to assume that it is just a retelling of the past events that takes place after Chapter 28 by Tassel far in the future after he is revived once again. That would just explain everything.