@schlessera , do you have any thoughts on this?

@iandunn Thanks for the work on this. It would certainly be nice to have autocompletion for URLs.
The main difficulty here is obviously the scalability. How can we make sure that this doesn't just block WP-CLI for a few minutes on a large network?
One approach could be to add heuristics that try to guess whether the approach would be fast enough or not, based on metrics that are quick to fetch and evaluate.
Another approach could be to launch the "site gathering" in a separate, monitored process with a timeout.
We should discuss and test approaches like that to ensure that autocompletion is kept responsive enough even in some worst case scenarios. Responsiveness is very important for autocompletion, otherwise it quickly veers from help to hinderance.
Happy to discuss here or help unblock.

add heuristics that try to guess whether the approach would be fast enough or not, based on metrics that are quick to fetch and evaluate.

How about something simple, like only enabling autocompletion if wp_is_large_network() is false?

Combined with caching an alphabetized list of domains/paths on disk, that seems like it'd be performant. If it's not, we can always drop the number to 5,000 or whatever is performant.

Psuedocode:

on wpcli shutdown
    if wp_is_large_network
        return

    if cache older than 12 hours || # of sites has changed since last cache generation
        launch background process
            wpdb->get_results( SELECT domain, path FROM wpdb->blogs ORDER BY domain, path ASC )
            write to ~/.wp-cli/cache/url-autocomplete.txt

That way the only potential bottleneck for the user is processing the cache file:

on url autocomplete trigger
    if wp_is_large_network
        write "can't autocomplete with more than wp_is_large_network() sites"
    else
        run autocomplete using cache file

Proceeding with #5594 for this repository. I've captured this PR to https://gist.github.com/danielbachhuber/f825f836ba8d9ed5270a8f1eff0e5d3d in case this PR is auto-closed or broken in some way.