It has to be a new IP. It's clear Killzone isn't the big draw Sony hoped for. Their talent would be best used for something fresh. Hopefully they can get some scenario writers onboard, I always felt the shallow narrative of Killzone was the main reason it never created a big following.
Speaking of which, what have Heavy Rain devs been up to since, well, Heavy Rain?
They worked on developing new performance capturing pipeline with Kara:
http://www.youtube.com/watch?v=G0KTUysrwgQ
And then they started making Beyond: Two Souls
http://www.youtube.com/watch?v=mOWN7d0125o
alfa gameplay, unfinished UI
http://www.youtube.com/watch?v=QbmcfJnjty0