I've had that thought before too, but until I just read this now I've never thought about it before but... we could just make a wireless T9 keyboard. It could be about the size of a car's keyfob, basically all battery and 3x4 grid of tactile buttons. And so all you do with it is hold it in your off hand and type with your thumb, that's it. But it's a Bluetooth keyboard that just understands T9 and works with any phone. Or even tablets or laptops if you wanted to pace around while typing.

Like, there are chorded keyboards and stuff like that, but I don't know I've seen a thumb-only phone-layout wireless keyboard before...