We want to make the core search module a better framework/API for integrating different search back-ends into Drupal.
The search_api module has an architecture quite different from the current search module. By being in contrib, it is also able to provide Views integration. We need to compare the approaches taken by search_api to the D7 core search module and develop a plan of attack for Drupal 8.
Look at search, search_api, solr, Lucene API, and other contrib search-related modules. Come up with a proposed architecture that would be more modular than the current core search module, combining elements from existing modules, and building a framework that the existing contrib modules could connect to and use rather than fight against.
We would also need to provide a default search back-end in core, which could either be the current database-based back-end of the core search module, or the Zend framework's PHP lucene implementation. Core should also provide a framework for facet blocks, sorting, etc, that can be leveraged by all the back ends.