Here is a 9to5Mac post on what they like to see and conceptualized the features for iOS 10 for the high-end iPhone 7. One that I would love to see is the picture-in-picture feature that is only available on the iPad. The iPhone 7 Plus would like have the same screen size at 5.5" as the iPhone 6 and iPhone 6S Plus. There is more than enough room on there for a diminutive screen for streaming my Youtube, Amazon Prime, and iTunes videos.
Why not the 4.7" iPhone? Well, given Apple's past on making features exclusive for newer and bigger devices, it would not be surprising if Apple makes certain features available on select devices if only to up-sell them - resulting in higher profit and revenue. It's not to say that this is always the case but I reckon features like PIP would be available only on the bigger screen iPhones because they would allow more room for operating an app while still have a small windowed video going on. If anything, Apple might also make it available on the newer iPhone 7 with the justification that it will have newer, faster, and more efficient CPU and other internal upgrades.
How would this affect play or workflow? These days, my iPhone 6S Plus has been relegated to the prominent role of an iPad nano while the SE has taken up duty as the main daily phone driver and my workout machine with the Apple Watch attached to it. And while I'm in a coffee shot or whenever the opportunity arise, the 6S Plus would serve as my writing machine, game playing, and video watching.
Of course, if you have a very small screen like the 5.5", let's face it, you're gonna be missing a lot of action. And if you're like me who has word captions on, you're not going to be able to read anything. Even now, I can't make out the words when I'm in PIP mode on the iPad mini. But a lot of time, if I'm playing Star Trek, Stargate, or a movie that I've seen before or a video that the image quality is that important, I would not mind. A lot of time, I just like that it's playing in the background.
The issue that comes to mind is the battery life, another reason why if the PIP feature is ever made available to the 5.5" iPhone, it would only be on newer and efficient iPhones. I'm guess at the very least, the iPhone 6 Plus would be out. Maybe the 6S Plus but I highly doubt it. If you want the PIP feature, you'll probably have options: get the newer iPhone 7 Plus or hope that that someone in the jailbreak community will find a way to enable PIP in older iPhones including smaller screen ones.