We could see a new Facebook operating system for mobile devices, so the announcement could showcase a Facebook OS running on a number of devices. With so much of Facebook's growth coming from emerging markets, where high end iPhones and Android handsets are more rare, this could be designed to enable Facebook and its various apps to work better on lower-end phones.
Facebook's growing mobile use includes tablets, so Facebook could extend its 'Messenger' app, to the iPad and android tablets. Eventually we should look for Facebook to introduce video chat—like Apple's"Facetime"—for tablets and smartphones. But if it's a tablet-related announcement, it would likely expand what Facebook offers for smartphones—group messaging and photo sharing, as well as the voice message feature—to tablets.
Something Search Related
Zuckerberg and Sandberg have both discussed the massive potential in search. The next step Facebook would announce in this arena is a better way to search within Facebook. Then eventually we'll see ways to use a Facebook network to deliver targeted search results from across the web.
Topeka Capital markets estimates that Facebook would generate "close to $5 billion in annual revenues and $2.5 billion in EBITDA from a social search engine." I don't think the search engine will be revealed Tuesday, but I would expect more news on it in the next few months.
(Read More: Here's What Facebook May Unveil.)
Ad Changes and Expansion
Facebook has been working on a number of advancements for its ad business, including video-ads and an ad network, to deliver targeted ads to sites across the web. We could hear about better rotation of ads, so people aren't bored by the same ads over and over.