Special:NewPages has a way to filter by user which offer suggestions based on the first few letters. Unfortunately, the suggestions are ordered alphabetically and that makes them useless for a large wiki and not very uncommon username.
Instead, you could use the the output from ElasticSearch, which are sorted by relevance under a much more useful algorithm.
Which function of ElasticSearch did you mean? Typing User:.. in the search box? That only handled existing user pages (relevance based on linking), not all existing users. That is not helpful.
At the moment it is using the api list=users function, no opensearch requests
Yes. Alternatively, "User Talk" should give more results for wikis with a welcome message
It is vastly more helpful than presenting an alphabetical list, especially for users with characters that are not normally on the keyboard of the wiki language. For instance, looking for "Gik" on ro.wp gives lots of users with no edits, but does not show "Gikü", one of the users with the most contributions (and new pages), which also happens to have in the name a character that is not on the Romanian or English keyboards.
I totally understand that ElasticSearch only returns pages that actually exist, so perhaps a new solution must be implemented, such as ordering the users by number of contributions or something similar. In my view, this would greatly increase the usability of the page (and any other page using the same code).
At the moment it is using the api list=users function, no opensearch requests