That looks pretty cool, and others must’ve thought that as well because it’s sold out.
Some of the immediate things that came to mind were making a case that integrates the phone with the keyboard; and a small battery to extend the life of the phone since the keyboard adds to the formfactor anyway.
Bonus points if this creates a market for a small phone again (iphone 12 user here).
Bonus points for e-ink-based keys.
Edit: Realistically I’d be surprised if something like this doesn’t already exist.