Guerrilla Games is busy working on a significant four-player co-op expansion to its PlayStation 4 launch title Killzone Shadow Fall - but it's also busy working on its secret next effort on the console.

According to the Dutch developer, its next game will benefit from its creators having had plenty of experience with the finished version of Sony's console. It was a different case, of course, with Shadow Fall.

Speaking with Eurogamer, Guerrilla producer Poria Torkan said making a PS4 launch title was particularly difficult because the developer was working with ever-changing hardware and software - even though, as a Sony-owned studio, it was as close to the console as could be expected.

"The best metaphor for making a launch title is it's like driving a race car while the car is still being built around you," Torkan said.

"You're working towards a goal, but in the meantime the hardware itself is also coming together; the libraries and the controller and everything."

Because it was shooting at an always-moving target, Guerrilla chose to focus on fewer features, with the likes of co-op failing to make launch.

"You need to be focused on the things you want to do in order to be able to do them properly," Torkan explained.

"So for us it was important to have a robust, chunky, sizeable campaign and at least tell the story we wanted to tell and have the gameplay mechanics we wanted to have, and also have a good multiplayer experience, with enough levels available at launch and more coming after.

"Within the scope we had, and being a launch title and having new hardware and having to re-factor the engine, there just wasn't enough time to also do a co-op.

We did announce it and say Season Pass holders would get it for free, so we did say we were going to do it, but we only started developing it after Shadow Fall launched. It was already planned, but not how it would be, or which type of co-op it would be. It wasn't something we were already working on before we launched the game. We started with, 'okay, what kind of co-op are we going to make?', after Shadow Fall was already out."

As well as class-based four-player co-op against waves of enemies, Intercept adds jet packs to Killzone Shadow Fall, a feature some were disappointed wasn't included in the main game despite being one of Killzone 3's best mechanics.

"That's the type of thing where when you're doing a launch title you need to focus," Torkan said. "Obviously you want to have the jet packs and the driving sections and the mechs, but we were already re-factoring our engine and there was a new networking module. If we were also doing this we were going to be too diverse. But now we had the time and proper purpose to bring it back."

Killzone's Intercept expansion apes Shadow Fall's multiplayer, which runs at 960x1080 resolution. It uses an innovative upscaling technique called "temporal reprojection", which combines pixels and motion vectors from multiple lower-resolution frames to reconstruct a full 1080p image.

Eurogamer played a four-player match of Intercept last week and the demo ran at an uncapped 30 frames per second, with most levels running at between 40 and 50 frames per second. Torkan pointed out that Guerrilla is still working on the expansion, due out in June, and so it may add the ability to lock the frame-rate to 30, as it did with the main game in February.

"Since this is a co-operative environment against enemies and not competitive, for us it's much more important to have enough power for the AI commander to be able to do stuff, to be able to have up to 30 or 40 enemies on screen at the same time, and at the same time be able to do proper, rich set dressing and effects and environmental things, and mortar blasts and four people with jet packs flying around," Torkan explained.

"It was more important for us to have that kind of stuff with the visual fidelity that Guerrilla is known for than to have it at 60 with fewer effects. In my opinion it doesn't play sluggish."

"It was more important for us to have the visual fidelity that Guerrilla is known for than to have it at 60 with fewer effects." Killzone Shadow Fall producer Poria Torkan

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Torkan added that he was happy with the launch of Killzone Shadow Fall, which has sold well amid a generally positive reception from critics.

But he pointed to the idea of Killzone "as a service", as so many developers think of their products these days, and the fact Guerrilla has added to and improved the game post-launch with patches and new maps.

"We're getting to a point in the industry that most studios see the launch of a game as a starting point more than 'we're done now'," he said.

"If we look at where Killzone Shadow Fall multiplayer is now compared to what it was when we launched, the amount of patches we've done, the amount of features we've added, from simple things such as clan support and the ability to have voice chat, to more intricate things such as weapon balances... we have systems like user voice, where the community can vote on which type of feature they want to prioritise to get worked on.

"We're not the only studio doing these kind of things. We try to leverage community feedback as well as telemetry we get, like, 'Hey, wait, this secondary attachment is getting way too many kills compared to all the other weapons.'

"Games are becoming more of a service. So in that sense I'm happy with how it went down, because I think we had a solid base when we started. Luckily it's selling really well. And we've continued to support it and are continuing to support it."

Guerrilla is certainly backing up this claim. Post-launch multiplayer maps are free to download, and the third one, the Canyon, launched just last week. Torkan said there are "many more in the pipeline" - and all will be free in an attempt to avoid splitting the Shadow Fall active userbase.

While the Intercept expansion will not be free, it will be released as an add-on and a digital standalone, so those who have not bought Shadow Fall can buy the co-op seperately. "We're really happy with that," Torkan said. "Some people don't like to play competitive multiplayer but do enjoy a co-op game, so maybe for them this will be of interest."

Intercept comes with four maps, but six will be added post-launch - free to Season Pass owners. "The plan for post-launch in terms of support is similar to the plans we had for competitive multiplayer," Torkan said.

"We will keep patching and polishing. We plan on extensively supporting it."

There is also another multiplayer expansion planned for Shadow Fall. This won't be the same format as the recently released Insurgent pack, but will be "in those terms". "We have many maps in the pipeline. It also depends on how co-op is received. If a lot of people like it obviously we'll be more inclined to investigate other options."

"If you compare Shadow Fall to the first games on the PS3, so if this is like the first generation of games which are coming, God knows what they will be like in four or five years."

In Intercept you play as one of four classes: Assault, Marksman, Medic and Tactician, each with their own special abilities.

Beyond Shadow Fall, Guerrilla is working on a new intellectual property for PS4, rumoured to be an open world role-playing game. This game has been in the works for some time, although Eurogamer understands next month's E3 trade show may come too soon for a reveal.

Back in September, Shadow Fall lead designer Eric Boltjes told Eurogamer this new game is "completely different to Killzone". "I don't want to say anything about it right now," he said, "but as a studio we do want to keep it fresh."

Similarly, Torkan kept Guerrilla's cards close to his chest. "It's really awesome!" he said. "It's exciting times. If I even say one word people will kill me."

Whatever the case, we can expect Guerrilla's next effort on PS4 to go one better than Shadow Fall - at least in technological terms.

"If you compare Shadow Fall to the first games on the PS3, so if this is like the first generation of games which are coming, God knows what they will be like in four or five years," Torkan said.

"Compare the first games that came out on the PS3 to The Last of Us or GTA5. It's amazing. That's the thing. We had two-and-a-half years to figure out what we wanted to do with our engine and one-and-a-half years to figure out what we wanted to do with the hardware. Imagine when comparably technically advanced studios get four or five years under their belts with the hardware.

"We'll get our chance!"