The completion facilities offered by readline and similar libraries are very useful for command-line users, but they aren't very easy to discover -- particularly when the completion rules are complicated, and the user may not know that their shell is able to completion what they're currently typing.
I'd like it if readline would show the current completion in a pale colour
after what I'm currently typing -- so I can see instantly what would get
completed if I hit Tab. This would invariably add some extra overhead, but in
many cases (such as completing filenames in a directory, or command-line
options from a parsed --help message) caching the results of the
lookup the first time would get around this, and the completion lookup could
start running in the background as soon as a completable word is detected.