On the Ubuntu side of things it is a full "Linux" stack. The code-base is public and available, and the community is open.
On the application front you can develop HTML5 or native (QML) apps. There's more info on developer.ubuntu.com. You can easily side-load applications etc.
The hardware thing is basically the same situation as with FirefoxOS. So that might be to your taste or not.
It's quite possible the ubuntu sw/hw will prove to be interesting to fork in order to make hybrid device as well (even if Canonical has abandoned that vision). But fos devices are already available. Either way I didn't mean to imply the ubuntu devices wouldn't be open (I fear they might lean that way, like Android and Jolla - I hope I'm wrong).
On the application front you can develop HTML5 or native (QML) apps. There's more info on developer.ubuntu.com. You can easily side-load applications etc.
The hardware thing is basically the same situation as with FirefoxOS. So that might be to your taste or not.