I don't think I understand what you're
getting at. It's been decades,
I think, since I last saw / used open mode, which is what I thought
you wanted, so I think I'm confused.
Sorry if I wasn't clear. I miss open mode as vim doesn't have it, and
I'd sometimes find it useful.
Separately, I use ed regularly, and have though about its `compose a
command, submit it', model being extended to commands that act upon a
cursor that's within the line, just as `.' is the line within the file.
I don't mean readline editing of the command, nor of the current line in
the file. More what one might see on the RHS of `:map Q ...', but in
the style of ed.