Android 4.4.3 KitKat is next upcoming update version from Google. Developer LlabTooFeR has confirmed this from a post on twitter. It says that Android 4.4.3 update is under development. Google has not given any information for release date, we cannot comment anything on its release date. Other than this, if you check log file then you will also find Nexus 5 running the update.
Currently Nexus 5 is facing problem with its camera and battery and so it is expected that Android 4.4.3 kitkat update will fix this issue. Other than this, we cannot say what else we will get to see. There are many background applications running in device like Skype and that interrupt camera app service by accessing it from its background service. Such access to camera app leads to overall system lag and reduced battery-life. Primary solution for it was to uninstall third party applications and reboot the phone. Earlier, Google released Android 4.4.1 update to solve this problem but it failed to do so. Till now, we just have Nexus 5 in update list. There is no information that if update will be available for other devices as well. If we check the last KitKat software version KOT49H (Android 4.4.2) then it was released only for a few devices that includes Nexus tablets and smartphones.
Apart from this, there is also rumored news that Android 4.5 will be unveiled by this summer with a few new nexus devices. HTC is also planning to release their next Nexus tablet by the end of third quarter of 2014. These all are rumored news and so we cannot be 100% sure for these.