My bet:

A new model for the viewer involving a main viewer totally rebuilt, including ‘plug-in’ options much like Firefox so that it can be scaled down or up depending on use models, and the ability to “tear-off” applications such as chat (the SLim part) or scripting so they can run as desktop applications (think an AJAX or AIR Second Life), an API for phone-based viewers, a browser-based API, all of it linked in to a new “social networking” platform - think SL Facebook.