Rather than phone, it would make more sense to make a UWA instead and could have Flow on the desktop along with Xbox One.
To me it stinks its not on the desktop, when I'm sitting at my surface I find it **bleep** distracting to have your phone notifiations goes off, have to stop and pick up the phone, unlock, reply, then switch back to surface.
Wunderlist is a great example can add stuff on my iPhone, then when I'm using my Surface and update list without having to pickup my phone.
I like Flow and just started to use it, but while writing this 2 push notifications came in and I had to stop typing to check them.
Also be neat to have Cortana integration, etc. I mean Windows 10 and Windows Mobile 10 have been merged isnt the push notifiation service part of windows now?