Notifications when friends come online ... - What? The PS4 doesn't have this?
PS2/PS1 classics - Yes
Download avatars on PS4 ... - As in, custom avatars? Then, Yes.
Custom Backgrounds - Yes
Increase max no. of people in a party ... - The most common arena/Pvp team size is five. So at least five, I guess.
Change of PSN ID - Yes
... Everything else is not worth investing time in until the ones above get priority, in my opinion. The ones I chose, I believe will benefit Sony and the entire PlayStation community the best. In other words, [Custom Backgrounds & Custom Avatars], our profiles will be much more "our selves" in a sense. Having PS2/PS1 classics and not only on PlayStation Now, but as downloads from the PlayStation Store will of course (financially) benefit Sony. More people in a party is always better, but if a limit must be held then five is good enough, I believe. Change of PSN ID... I understand their reasoning to avoid grief'ing, but I'm sure technology has come far enough.