Intel has released a chart detailing what the next wave of ultrabooks can include, giving a pretty good hint as to what the next round of machines might look like.
For example, a VGA connector won't fit into the 15mm super-slender laptop, and as far as memory goes, it has room for a 2.5-inch "ultra-slim" drive. Nor will the smallest version have "standard thermal solutions" or the largest 18650 batteries. Of course, there's more breathing room as the ultrabooks increase in size, to 18mm and 21mm, which really could be packed with so much power that it's more or less just a skinny laptop.
The chart (above) is colour-coded, with green referring to what will work, yellow meaning that it "may not fit" unless modifications are made, and red signifying that the part will not fit. It also should be noted that Intel calls this an "engineering estimate", which means the actual machines could stray from what is listed. [CNet]