Hello,
Our IntelliPrompt completion list should already support that kind of thing. When your completion provider opens a completion session, it can pass in a text range with within to initialize the selection. In a case like what you describe, you'd pass in the text range of the "x" identifier. If any item starts with "x", it will be automatically partially selected. Or if there is an item that only has text "x", it will be fully selected.
Our documentation on completion lists is here and our samples show many use case scenarios in them.