Do you plan to support the equivalent of ~/.inputrc to configure editing options (specifically vi editing mode)?
https://www.gnu.org/software/bash/manual/html_node/Readline-...
https://www.gnu.org/software/bash/manual/html_node/Readline-...
One thing I underestimated was the importance of having all readline key bindings available. People are really sensitive when certain functionality that they are used to is missing. (And you've no idea how much functionality there is in readline until you have to implement it.)