Two more I have in mind: Supporting a GNU sed extension of taking both a starting offset and g with s///Ng - replace all matches starting with the Nth one. I have this mostly done; I'm not sure how it should interact with the current toggling behavior of sN and sg, though.
Adding an ex-like range to ! that sends the addressed lines to an external command and replaces them with its output. I don't think there's any good way to do this now that doesn't involve multiple steps and a temporary file.