The current auto-completion behavior across all wikis is a search against page titles. This is highly successful for encyclopedic content, such as finding the page about a specific topic, but is less than ideal for constructing a full text search query. We suspect auto-completion of search queries could be a significant improvement to the search interface particularly. The initial exploration and implementation will focus on commonswiki and multimedia search, where we expect the current autocomplete is not nearly as successful.
The primary concern to address is privacy. While query completion could be implemented using a variety of methods, implementations based on submitted user queries are simpler to build and are documented to significantly outperform models built from dictionaries or content, as long as sufficient user interactions are available to feed the system. User queries are PII and should not be directly released. Since providing the query completions to users is a de-facto release we need to deal with this. Typical approaches involve removing the long tail of search queries.
The second concern to address is NSFW content. Any sampling of a few dozen search queries is bound to have queries that refer to pornographic content. In initial explorations of the most popular queries starting with various letters, many of the top 5 query lists from commons have results that fall in this category. While we don't take a particular position on the appropriateness of content, we do have prior experience with concerns from communities when search returns nsfw content to seemingly unrelated content (see `gedit` -> `genit(al)` and commons search disabled in enwiki sidebar). It is unclear what the appropriate way to address this is, but likely some consultation outside search platform will be required.
As an initial exploration/implementation, we should try to keep things simple.