Guerilla Games is fully owned by Sony, meaning Sony wouldn't be risking much by giving GG's
270 or so industry veterans full access to PS2's Forgelight engine.
The last Killzone was kind of meh, and Killzone as a franchise really doesn't have a great chance to defeat
COD or
BF at the moment. The guys at Guerrilla Games have basically made Killzone sequels nonstop for the past decade and PS2 could be a welcome change of pace. Killzone and PS2 share some similar styles and traits (modern warfare + scifi), and so it wouldn't be too much a departure from what they're used to. Guerrilla's experience with consoles would also benefit the launch of Planetside 2 on the PS4 as well.
Planetside 2 presently
leads the MMOFPS market (such as it is), and can hold onto that position as leader if it gets more than its current skeleton crew. The current development pace by the small PS2 team suggests the Playstation 4 launch with all the goodies (continental lattice, new continents, ANTs, resource revamp, missions/directives) between November 2014 and March 2015.
Planetside 2's client/server hybrid isn't overwhelmingly difficult to replicate, and so I imagine a competitor will arrive on the scene probably by early 2015 (Could be Titanfall MMO, a Halo MMO, COD MMO, new IP, who knows).
So... make it so?
