KitKat. It's coming soon. Although we still don't know exactly when its coming. We're expecting it to be announced any day now. We've heard a few rumors of Android 4.4 using the new 3.10 Linux kernel, which would bring all kinds of improvements to the OS, over the past few weeks there have been a number of commits in the android-3.10 brand of the kernel/common project. Most of us already know this, but I state it anyways, this is the branch of AOSP that houses all the code for the kernel. The 3.10 branch in AOSP is pretty solid evidence to believe that we will see this in Android 4.4. There are also architecture-specific commits for 3.10 in the kernel/tegra project, which also points to development for the Nexus 7 (2012).
Since the beginning, Android has kept their kernel up to date mostly with Linux, aside from the usual 4-8 months of being behind due to development and release schedules. In fact, Android 4.3 was the first version of Android to not get an update to the kernel. Android 4.3 remained on the 3.4 codebase, as Android Police points out. Which makes it easier to see that Android 4.4 - KitKat is due to get caught up, and the development team certainly won't want the kernel to get even more outdated. The 3.10 Linux kernel was released just 4 months ago.
So among the many other changes we are expecting to see in Android 4.4, soon, we should see an update to the 3.10 Linux Kernel which will make the OS a bit more stable. Along with brinigng some improvements to performance, nad battery life. We should also see reliability improvements to the ext4 filesystem, better support for big.LITTLE ARM and 64-bit architecture. Which we did hear that Andorid 4.4 might support 64-bit, after Apple pushed out a 64-bit chip in their new iPhone and iPad this year.
At this point, this is just about confirmed, but as always we'll wait for word from Google before saying it's official or confirmed. You can take a look at the commits down below.