On a large WordPress Multisite project (multiple brands / products), we are currently using Swiftype (Elastic) to power our site search and have only really scratched the surface in terms of possible features.
We'd like to move away from the current (slow as we're indexing ~ 100k pages) site crawl across multiple 'engines' to the API version - consolidating the index under one engine, with defined conditions around the displayed search results depending on the current view (e.g. only display pages with xx attribute on the .com vs [login to view URL] domain).
Beyond that, there's scope for wider development - an advanced faceted search with different content types, plus building in commercial aspects (sponsored results).
Scope for this to be an ongoing project for the right person!
Migrate all existing engines (4) to a single (1) engine via the API version of Swiftype
Build faceted search (inc. options for commercial weighting / boosted results)
Build conditional display - based on content types and/or current user location (domain or website section) or geo-targeting.