The downfall timeline feels like the "canon" timeline to me, and based on the focus Nintendo gives it I bet they consider it the same way
Given how far down on the timeline botw is though I think they might start to find ways to push new games into other parts of the timeline
Yeah, the fact that it's the original timeline ATLEAST from a meta perspective due to the first four games being on it(before OoT/the timeline split was a thing), the fact that it has the most games, the fact that it's the timeline where Ganon, the main villain of the franchise, appears the most often as a result of him becoming the eternal scourage of Hyrule, etc. makes it a candidate as the ''main'' timeline. I think calling it the ''canon'' timeline isn't the right word, though; all three are canon and valid, hence why they're included in the official timeline. It's interesting that every new game since the unveiling of the full timeline in 2011 seems to be in the DT, though...
I'd like to see a new game set on the Child Timeline, maybe one last game featuring the Hero of Time set after Majora's Mask.
I'd prefer a direct sequel to TP with the same Link, since we didn't get one and TP ends with him leaving Ordon. Maybe a new game between TP and FSA with Ganon I coming back, since he didn't get Triforce'd to death at the end of TP like he did in TWW, there's potental for nearly an infinite amount of games with him returning on the CT, not as the mindless madman like in the DT, but with his wits remaining. Maybe a game can happen down the timeline with Ganon I being killed off permanently in a special way to make it clear he's gone for good so that Ganon II can take over in FSA.
I wouldn't mind seeing a new AT either, maybe with a new recurring villain for New Hyrule being established, as well exploring more of New Hyrule's customs and how they differ from the original Hyrule.