They could just end it by the time they reunify Hinomoto, but yeah it will take a bit of imagination from the author, which I don't mind. They've already been deviating for a while anyway, no? The variation I'd hate the most is if Nobunaga also ends up in the modern age, tbh. I don't mind Shizuko dying and being flung into a future where Nobunaga accomplished his goal, that'd be interesting I think. You get to skip a lot of the minutia that way, too. Everything ending at Honnoji would feel like everything so far had been for nothing.
From ch1 it was always about Shizuko dreaming up how to win with Sengoku warlords, so it'd be the perfect way to end it. Sometimes subversion is unnecessary. Alternate reality would really test the author's skills, though.
I haven't read the novel version, but based on the manga version, it just feels like the author is sort of lazy. Many things aren't explained very well and Shizuko often wins easily. I don't mind it, but on the other hand, it's the reason why I don't see the author putting much effort past the Honnoji incident and try to end it as soon as the story reaches that point.
It is true that it has been deviating from history for a while, but it still sort of follows some kind of timeline. Without that timeline, the author would have to get a bit more creative and it won't be very Sengoku anymore. Come to think of it, that might not be so bad. But...I just don't see the author capable of doing that and still have an interesting story.
The scenario that I came up with is what I think would most likely happen with a lazy author that just wants to finish with a happy ending as quickly as possible while keeping true to the theme of the story. It's not necessarily what I want to happen, but it's what I predict will likely happen from my years of reading manga. In fact, I think I sort of dislike it, because it's kind of predictable and I much rather be surprised with a totally unpredictable ending.
In my opinion, the worst type of ending would be if everybody survives the Honnoji incident like it's nothing special and the story stops right there while the narrator says something along the line of "And they lived happily ever after". And no further detail is given. I mean, that's great for Shizuko and friends, and maybe many readers will like it, because their favorite characters didn't die, but I just think way too many manga follow that style of ending or some variation of it, and it's really boring.
I'm fine with Nobunaga and Shizuko unifying feudal Japan and all of her hard work doesn't go to waste, but only if it's done well and the author doesn't just end the story abruptly with "Oda and Shizuko unified Japan, her inventions created a utopia, and everybody lived happily ever after" without telling us how.