The weird thing about The Assembly is that they exist in a kind of canon limbo-land. 343 has suggested that it might have been an in-universe work of fiction or fabrication, but in a way that they could shrug it off as The Assembly hiding themselves, if they ever do think of something to do with them. If not, they can ignore them.
Personally, I honestly doubt they'll appear in the Created storyline, at least directly. Maybe in an obscure piece of intel or terminal. It's just a too obscure piece of lore to be brought up out of nowhere in a game or novel.
I agree that, given the tiny hint of 343's stance on the Assembly we have, it's highly unlikely, but I disagree that it's too obscure a piece of lore to include.
The Created storyline is practically tailor-made to include them, if 343 wanted, even if most people don't know who they are yet. This storyline is all about AI, it's all about how human-made AI treat and relate to humans, it's all about AI running the galaxy. That's so solidly Assembly territory that if 343 isn't actively against the notion of them existing it would make zero sense at all to leave them out now. They could easily have a big enough impact in their story presence to rise above obscurity, if they came out of the shadows to fight for or against Cortana.
I could definitely see Chief doing missions for a human force that depends on Assembly AI to help them fight back against the Created (someone needs to help the humans drive their space ships, etc). Roland could be revealed as being one of the Assembly, giving us a simple thread to connect to the group for those unfamiliar.
There has to be some
kind of friendly AI presence in the story of Halo 6 and onward, after all... at least there better be. Probably won't be the Assembly, no, but it would make a lot of sense in the canon (as much as the Assembly can make sense in the canon) to see them pop up, in my opinion.