What do you want your desktop search to do? The two big things are obviously message store, then file system documents.
Miscellaneous things follow, like integration with team workspaces, IM sessions, etc. But the most important next step, after message store and file system documents, is to access databases. Database access is a huge black hole for search engines.
... David Ferris, with thanks to Persistent Systems' Gautam Malkarnekar