Splash Damage could easily be folded into SIE to work on existing or new IPs.
Resistance could work.
Already a large number of units over 5 games, and fairly easy to add to it.
Resistance seems to be the better fit for the RTS genre than Killzone now that I think about it.
Also, another interesting thing to consider, is Leyou's connection with SNK. Could it be possible for Sony to acquire the rights to this team too if the Leyou acquisition goes through? It would be another versatile addition to the SIE umbrella.