[user104658]

Make it powerful enough for proper PS1 and 2 emulation with upscaling / anti-aliasing options etc., and keep the chip architecture the same so that PS4 games just work natively, maybe boost some framerates from 30 to 60 with patches.

Unfortunately PS3 is going to be the black sheep for quite a while longer because of the weird custom processor; it's pretty much impossible to emulate without actually including the physical chip (which would bump production costs way up).

However... There are already PS4 versions of a lot of the real classic PS3 titles so it shouldn't matter too much.