Well, first of all, remember that Google I/O is more than just about Android, and definitely more than about the Nexus line. There are Chrome (and Chrome OS), search, Wallet, Google Plus (and Hangout), Maps, coding for Chrome OS and Android, Glass, and any number of other products and services.
I doubt very much any new version of the Nexus hardware gets announced. I don't think Google wants to confuse users in a shorter-than-one-year timeline. There may be things on the software end, like Android 5.0.