It is literally the aspect ratio of every iPad, though 😅

Something like 16:10 or 16:9 would make it very square when folded. IMO the most satisfying aspect ratio would be √2:1, like A4 paper, so that the aspect ratio when closed and open is the same.

The problem with that is that nobody wants an 8:9 screen for the folded in form factor

I think the durability is the limiting factor in regards to foldable tech as it stands right now

I'm the kind of contrarian who thinks we should have stuck with 4:3 in general, it's a better framing for a lot of content and it also makes the vertical video problem a lot less annoying in my view.