Almost definitely on the docket for the event will be refreshed versions of the iPad Air and iPad mini. There hasn't been a ton of rumors around what changes to expect, but they will likely be mostly internal updates, like the new A8 chip, as well as the likely addition of TouchID sensors on both Apple tablets. The report also says the newest version of OS X, Yosemite, will be released right after the event, and that will bring support for the Continuity features between MacOS and iOS.
The real question surrounding the announcement is whether or not the rumored 12.2-inch iPad Pro will be part of the event. Apple had promised new product categories announced this year; and, we have gotten those with the 5.5-inch iPhone 6 Plus, and the announcement of the Apple Watch. There have been rumors the larger iPad won't be released until next year, but that doesn't necessarily mean we won't see it announced now, just as we did with the Apple Watch.