Definetely check out Sailfish OS by Jolla, a company founded by ex-Nokians. They've only got two phones of their own (Jolla and Jolla C, latter very limited batch) and one half-failed tablet out, but they are collaborating with several other manufacturers to license Sailfish OS on others' phones. The most recent and interesting collaboration took a step forward as Jolla released instructions and files for installing Sailfish OS on Sony Xperia X. This is an official collaboration, so you'll have to pay for the OS image, but install it on your own following instructions (and using a Linux machine). Furthermore, they support fans porting the OS to other phones, so if you don't necessarily want official support and Android compatibility through an official emulator, you can look for fan-made ports. You can find a list of more or less supported phones here.