I think the PS5's killer features is going to be a combination of Alexa, Shareplay+remote play, VR capability and even better multimedia functions such as 4k blu-rays etc.
At least that is what i am hoping for.
I hope the controller will be innovated on and include more features maybe even better 3d pointer tracking to make it match up with the best PC VR solutions.
Full backwards compatibility with PS4 both digital and physical. Perhaps some emulation solutions for PS1+PS2 games although i am not holding my breath for that one since a lot of games will be flat out broken if it is just pure emulation and not done like Xbox one does it currently.
I am hoping to see freesync and the capability for higher framerates than 60fps outside of VR (currently only PSVR can have framerates above 60fps).

I know this isn't exactly a killer feature but rather a list of features. But i really think they are going to blow us away with a ton of features rather than just 1 big thing. Arguably Remote play is already a big thing in Japan for PS4 since it appears that it is usable out in the wild aswell as at home. We will see.

That is not how they run the PlayStation business. PlayStation Hardware is not the money maker. They sell consoles at break even or a slight loss. The profits for SIE come from Game Sales, and Network Services. 

I don't think they have to do anything additional to give them their own identity. The Home console is for those that want 4K/60, UHD Blu-ray, and Next Gen VR. The Hybrid is for people that are fine with 1440p/30 docked and 1080p on the go, and also have an Android or iOS device as well. The Phone would be for people that just want one mobile device that can deliver their games, while also keeping them connected to their priority needs. There is a place for all of them, and if Sony has a unified architecture across all of the devices it is very easy to support, and R&D costs are extremely low.

PS4 is not just one sku. PS4 has actually had 3 very different Sku's, with 7 revisions in total. PS4, PS4 Slim, and PS4 Pro are all different devices that had their own R&D. They share core CPU/GPU technology which, would be the same case with the PS5, PS5 Hybrid, and PS5 Phone. With a massively scalable architecture like Ryzen and Navi, it wil be possible for Sony to have all three devices. Thanks to the focus on scalability across game engines, it wil be very easy for devs to build games that support all 3 devices.

This business model would be massively simplified over previous gens. Just look at last gen to this gen. Sony was supporting;

PlayStation 3 - PowerPC CPU, Nvidia GPU, and PS3 OS

PlayStation Vita - ARM CPU, PowerVR GPU, and PS Vita OS

PlayStation 4/ Pro - X86 CPU, AMD GPU, PS4 OS

SIE was supporting 3 vastly different platforms, with their own CPU, GPU, and OS. They had to build everything for all three devices from the gound up, whether it be OS related, Network related, or Software related. They where also working with numerous suppliers.

The purposed scenario for next gen would be;

 PlayStation 4/ Pro -X86 Jaguar, AMD GPU GNC2, PlayStation OS

PlayStation 5, Hybrid, Phone - X86 Ryzen, AMD GPU GNC6, PlayStation OS

This is much simpler, much cheaper, and anything they develop is functional across all devices. Not only would they be able to support multiple versions of PS5, but thanks to PS4 using X86/AMD, they can continue to support it for years at a very low cost. With more options for entering the PlayStation ecosystem, they should only see an increase in Units Sold, and Total active userbase.

As of yet I do not live in 2020, nor would I believe that you could effectively lag free play a game at even 540p using satellite broadband on a plane.

Just think about the science of this from a latency point of view, at best both those solutions suggest you'll have maybe 5-10 satellites available to you, even at the speed of light the beam would need to leave your Vita, go through the router on the plane, into the antenna, travel to the satellite, bounce from there to their ground receivers, pass that data to your ISP and get it routed to your PS4.... and all the way back, in 16 hundreds of a second for 60fps, keep in mind that it takes light 1/8 of a second to fly around the world un-suppressed by tech which it would need to pass through.

At the end of it all those net options (suggest they can) offer up to 50Mb connections to a plane... or 6.25MB which would be fine to stream from a data point of view (not latency by a mile as shown above) but that would require the entire planes net connection to do it.

Also!

"Historically, earlier companies that have attempted to build satellite internet service networks and provide space-based internet connections have not fared well, as both services were hobbled by high costs which consequently attracted few users. Iridium SSC filed for bankruptcy protection in 1999 and Globalstar did the same in 2002.^[9]"

So yeah.... Switch works without requiring a system which bankrupt multiple companies trying to pull off what would be required for the Vita/PS4 link to work.

I'm not saying this sort of data link would never be possible, but you're talking at least 20 years out imo before it's commercially viable and available.

They would be 2 complementary devices.  One a powerhouse console while the other would be the Lite portable version of it.  The GO would cost as much or a bit less

It is the same eco-system, it wont affect any sales even if it costs less.

For now, remote play is absolutely useless to me. I need to be home to be able to use it and if I'm home, I have access to a TV (or monitor, if I'm really desperate - which hasn't happened yet). Not only that, but it also requires an accessory (for the controller) that's not useful for anything else.

I like the idea of a go version. 

I don't want a PS5 hybrid at all. I want Sony to keep releasing powerhouses. Hybrid is just a fancy and smart way to put handheld gaming on TV.