So what you would like is a keyboard covering half the display at all times?
The rockbox text editor is "line based" in the "viewing" mode the keyboard is not visible giving better overview of the text. When you select a line to edit, the keyboard pops up.
The currently available keyboard works very nicely and is very flexible, supporting any characters available in UTF8 and in any layout you like, if you want it to be replaced, the new one must do all that and have something making it worth the change. And the one in the "RockWord" which I assume is the one you meant, is very limited.