I would still guess it's a real longshot to get more than PS4 BC. At least now they won't remove it later on like they did with PS2 BC on PS3 since it doesn't need extra hardware. Though maybe we'll get partial PS3 BC like XB1 does for 360 where the more popular games get XB1 ports that download to your hard drive the first time you put a 360 disc in for a supported game. For PS2 I know the PS2 Classics they did for PS4 were upscaled and added trophy support, and you had to buy them off PSN. So I'd imagine that's the best you'll get for PS2 BC in the PS5. God would I love full PS2 emulation in PS5 though. PS1 I don't care since I can do that on my PS3 or my Vita. PS2 I care about though since component input is being quickly phased out of TVs and there is no really good HDMI solution for hooking up a PS2 AFAIK.
What I'm really hoping for out of PS5 is that the 2020 PS4 lineup gets 60 fps + higher resolution patches for PS5 that are free if you own the PS4 version of the game. It would make sense since PS4 has a ridiculously strong 2020 lineup but launch les for PS5 will likely be pretty crap just like they were for PS4 and PS3. I mean we still don't even know any launch les for PS5. So Sony could use the strong 2020 PS4 lineup as a de-facto PS5 launch library. Not like I'll ever get through all the great stuff coming out for PS4 this year by the time PS5 launches anyways.