Basically, I'm thinking of something similar to the Emacs (or shell) behavior. With "nix"-style completion enabled, if the user presses TAB twice when the completion is ambiguous, show a list of all options. This could be done with a minihtml hover, or (preferably) by expanding the panel to include the file list above the file bar (ideally with sidebar-derived icons, but really it doesn't matter).

As it stands, windows-style completion is almost the only useful one (despite its annoying behavior), since it's the only way to explore a directory structure.