Well, the problem is that you would need to bring the accessories with you, not just the phone - and these seem to be rather bulky to me. The Xperia Play at least was ready for playing out of the box.
But I agree that they could release a new Xperia Play with similar hardware than the ROG, just like the Play with built-in controls. Add a big battery and I wouldn't mind if my phone would be big and heavy, at least it would do so with a purpose.
While we're on it, I expect Nintendo to bring a Switch Phone around 2020 (without removable Joycons but otherwise similar in design to the Switch)
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454
The Steam/GOG key gifting thread: https://gamrconnect.vgchartz.com/thread/242024/the-steamgog-key-gifting-thread/1/