A new project named Maru promises to provide users with a full Android Lollipop experience on the phone and switch to a Debian Linux distro when connected to a monitor and peripherals.
A phone that is powered by Android and magically transforms into a Linux desktop when connected to an external display has been tried before. It was called Ubuntu for Android, and it was one of Canonical's earliest attempts at some sort of convergence between the mobile and PC worlds.
It never succeeded, and it was never launched, but the idea was working, and they had a preliminary version of it in a sort of functional state. It's not clear why Canonical dropped the project, but it probably had something to do with the hardware, which wasn't all that powerful three or four years ago.
Maru is trying to do a similar thing, but now the team behind this project has better hardware, and it should work, at least in theory.
For the moment, Maru only works for Nexus 5, and it's in a closed Beta. This means that if you subscribe, maybe you'll be given access. Unfortunately, it doesn't seem to be the kind of project that can work on anything, and support needs to be added to each individual phone model.
“Maru Mobile is built on the latest Android Lollipop. It ships with zero bloatware, so your phone runs snappy and has lots of free space for all your apps. Maru Desktop brings you true multitasking and desktop productivity in a lightweight package. Peek under the hood and you'll find rock-stable Debian Linux, so you can customize it to your heart's content,” reads the website.
The idea stems from the same observation. The phone is a fairly powerful device, so why not use it for other stuff than just for making phone calls and maybe playing a couple of games?
The only problem is that there is no mention of whether this is an open source project or it's going to be a commercial enterprise, but we'll just have to wait and see.