Twenty Thirteen will be the next WordPress default theme. It is scheduled to be bundled with WordPress 3.6. As you can see, the design is quite unique, unlike the previous default themes which have aimed to be more of a blank canvas (especially Twenty Twelve).
It’s kind of strange seeing a new default theme just after the release of Twenty Twelve, but I guess it’s better to get these yearly themes out closer to the start of the year then the end.