First up we should see the next version of Android, aka Jelly Bean for the first time in all its glory. Originally it was thought that Jelly Bean would be a major new retool of Android, but expectations have been toned down to the level of an incremental update.
Seems like that's what happened with Ice Cream Sandwich. I recall it was originally touted as a major new version of Android but then basically became an incremental upgrade over Honeycomb.
Whether Jelly Bean is truly incremental or monumental, given the state of Android the only way to get it any time soon is to buy a Nexus. That would be either a Nexus phone or a Google tablet.
The Nexus tablet is the other big unveiling expected at Google I/O. Rumored to be a 7-inch tablet for $199 to compete with the Kindle Fire, the Nexus tablet will be Google's first foray into the slate space.
While the Nexus tablet is expected to have good hardware, the primary expectation is that Google will assume full responsibility for updates once released. It seems Google can't fix the horrible Android update situation caused by OEMs and carriers so it's just going to handle the new tablet itself.
Speaking of Google I/O and Android updates, remember last year's big update alliance Google announced? Neither does Google, apparently.