Well, being realistic...
I see 10.7 hitting around this time in 2011. I can't predict features like Spotlight or Expose that they would add, but here's what I'd assume will be there:
Removal of Rosetta/Universal Binaries
Core Duo support dropped, making 10.7 a 64-bit only OS.
The rumored Marble interface (already, we see hints of it in Coverflow, Quicklook, inspector panels, the new dock's contextual menus, etc...)
Blu-ray support at the OS level (required for DRM stuff)
Greater Dock/Finder/Expose integration (if you follow the pattern from 10.4 > 10.6, to be expected)
At or around the same time, if it is not announced at the 9/9/09 event: iTunes X
Rewritten in Cocoa, 64-bit app (dumps PPC support, but apple will let iTunes 8.x sync with current/new iPods?)
Blu-ray support/playback
Much, much better handling of iPod/iPhone media and syncing
I really hope they take the SL approach and slim it down, it's really far too bloated for a music app