I don't know this market well, but you have me wondering. I imagine that most users of the phone would want access to some smartphone like apps - i.e. alternative messaging like Whatsapp/Telegram/Signal or some sort of music/streaming service. They probably want at least one e-mail client too, and possibly word processing.
When you get down to it, it essentially becomes a custom android ROM with a fixed set of approved apps. While brining back the keyboard phone might be noble, you may want to consider a separate 'light' OS service that curates what apps can be installed, since there is seemingly a demand for it.
As for the keyboard phone, one of the tradeoffs will be screen size. I have to figure that a keyboard will take up about 2.5 inches and even if you use a bezel-free screen, you're looking at, say, a 3.5-4inch screen. Obviously, if you are coming from a blackberry or flip phone, it's an improvement, but from a smartphone, not so much.