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.