Theories regarding the InuYasha sequel Yashahime: Princess Half-Demon.
I. Sesshoumaru & the forest fire
In the newest episode 14 it was revealed that an currently unknown demon lady named Zero gave the fire god Homura the order to burn the entire forest down where the twins were living. She asked Sesshoumaru if it was okay for them to burn the forest down (knowing that his kids were in there) but he walked away leaving the question unanswered.
It is pretty much unknown and strange why Sesshoumaru would leave the other demons to burn the forest down, but I think perhaps at that time he didn't have any other choice but to agree to it. The Tree of Ages said back then that Sesshoumaru and Kirinmaru walk the same path (with Sesshoumaru perhaps working under him / serving him or whatever..). Maybe Sesshoumaru was forced to join Kirinmaru to save Rin. The new lady, Zero, is also known as the Dream Butterfly Lady, so perhaps it's her who's keeping Rin imprisoned in the 'stasis' thing and robbing Setsuna of her ability to sleep and her memories - and Sesshoumaru is forced to obey Kirinmaru in order to ensure Rin's safety. Zero might be the only one who can wake Rin up.
The 'snake mommy' Joka, who also seems to work under/with Zero, said a couple of episodes before that she would take the silver and golden pearls from the half-demon girls and use them for a revenge against Sesshoumaru.
It might that in the meantime (during the 10 years) Sesshoumaru betrayed Kirinmaru & his underlings and Joka seeked revenge on him because of that. (I wonder how his current relationship is with Kirinmaru and his underlings).
It also might be possible that Sesshoumaru pretended to give Zero & Homura consent to burn the forest down and in reality he tried to save his kids during the fire behind the demon's back (either himself or with the assistance of Jaken & A-Un). Maybe the disappearance of Towa would later upset him, much to the point of betraying Kirinmaru and his underlings (despite being unable to bring Rin back) - perhaps he wouldn't know that Towa would end up in the modern world and he might have believed that Towa died during the forest fire.
Another theory is that Sesshoumaru would trust in his daughter's abilities to survive the forest fire (knowing that they've been given the pearls) hence why he would give 'consent' to the demons to burn the forest down. But.. maybe he wouldn't except that Towa would get sucked into a time-tunnel into the modern world.
Perhaps in the beginning he was more concerned about Rin than his daughters, but after Towa went missing it might have opened his eyes (and put him into a much bigger dilemma than before - Saving Rin or savin his daugthers ?). Overall, as a dad of two half-demons he's been put into a crappy situation (maybe much like Riku had described in the 14th episode - that a demon & human love relationship causes a lot of problems - maybe just like Toga got after he fell in love with Izayoi and he had to die tragically, giving a young Sesshoumaru a reason to despise humans and half-demons.)
Basically, Sesshoumaru became someone he initially hated and fought against. (the irony, lol) And now he is in the same situation as his father had got before he died. I just hope that he won't meet the same end as his father Toga, that would suucckk~ and I would cry a river ;-;
All that aside, from the new Yashahime opening it seems that Sesshoumaru will be fighting Kirinmaru, possibly alongside his daughters, and InuYasha's team.
^ That smile tho~ Is it Real ?? o.O (not an edit?). If yes, bruh~ he smiled at his sleeping daughters (so darn cute~).
The kimonos the kids are wearing (with their family pattern) ^. I'm sure of it that Sesshoumaru must have been watching over them after he had put them inside the (barrier protected) forest and gave them the new kimono's as they've grown a bit older. Maybe even a much older Setsuna received a bigger version of her kimono before she came to the human village (it's hard for me to believe that she'd been wearing the same kimono Sesshoumaru possibly gave her when she was 4yrs old until she'd come to the human village).
II. Setsuna's mokomoko
At an unknown time Setsuna appeared in the human village with a mokomoko around her upper body.
This could be a sign that Sesshoumaru might have tried to save Setsuna from the fire, or that he (or Rin) might have raised her for a certain amount of time (Sesshoumaru would give her a part from his mokomoko) before Zero would find out about that and seal Rin inside the 'stasis' thing and take Setsuna her memories away.
Also.. someone on Reddit came up with a theory that Setsuna might have known the human village before her memories were erased by the Dream Butterfly, so perhaps that's how she knew where to go to meet up with Kaede or the demon hunters - because Rin would have raised her there, but then.. idk, most people from the village don't have any memories (of Rin).