I just finished the campaign and I have to say, calling that abrupt is an understatement, at least in comparison to what actually happens at the end. What I'm trying to figure out is where the franchise can go from here.
 
Quick recap: 
Helghan just got completely nuked / irradiated / slimed, a bunch of the helghast fleet got destroyed, and it appears Stahl and his techno(viking)solders are the only ones left standing. There may or may not be a conventional invasion fleet already at Earth, and Sev and the gang are out in Helghan space with a possible means of transportation.
 
My speculation: 
Stahl will take control of whatever's left of the Helghast, pressing everyone and everything into his military industrial complex with a healthy dose of added mania. The new planetwide supply of irradiated petrocite (green stuff which is apparently deadly to everything except Rico) can serve as justification for their ability to rebuild. There might also be some random Helghast colony planets (I don't know the lore) referenced as supplying the additional bodies for said army. 
 
Back on Earth, I'm betting Guerilla will pull a Halo 2, with the returning heros meeting up with the initial Helghast invasion fleet that was launching near the end of Killzone 3, which has done disproportionate damage to Earth's forces thanks to the whole surrender bait-and-switch. After a prolonged battle against standard Helghast forces with the ISA making progress, maybe a third to halfway through the game, Stahl and friends will show up with a bunch of green ships/guns to mix up the gameplay and give the plot some drama. I'm pretty sure it will focus on Earth, since Vekta doesn't seem very important and Helghan is too radioactive for Sev to spend another game in his T-shirt (lead character redesign is a big nono). Besides, we've now had 2 games in a row on Helghan, and I'm guessing the art team is clamouring for a change.
 
The plot will again pay Stahl lots of attention, but this time he'll be extra-crazy as a result of all the shit that went wrong in Killzone 3. He was probably also gravely injured in the ending of Killzone 3 and will present at least one mechanical appendage, possibly becoming mecha-Hitler for a 1-on-1 final boss battle against Sev, as Stahl attempts to petro-nuke Earth. 
 
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So, how does that sound, folks? Am I missing something? Have an alternate theory? Any idea what will happen after Stahl arrives at Earth?

as someone who knows of of the story from bombcasts and reviews. Does rico die?

So the series isn't over? I am disappoint. 
 
I was expecting KZ to be a trilogy. Perhaps I was naive.

I would love a Hitler-like boss with 2 miniguns.

" as someone who knows of of the story from bombcasts and reviews. Does rico die? "

" as someone who knows of of the story from bombcasts and reviews. Does rico die? "

And now... Killzone 3: Recon.

That ending was kinda crazy/dumb. Like really, all of Helghan is gone? Just like that.....oh well. I guess they could go the Gears 3 route and have the green stuff turn Helghast into monster dudes who will go wreck shit as a last ditch attempt to win.

Yeah, i was very interested in the story and characters and then out of nowhere we launch a nuke kill a planet and credits. No wrap up no encounter between the main good and bad guys, no wrap up for Rico or the captain just kind of a setup for a sequel. Great game and even story but terrible ending its like they ran out of time.

As soon as continuity got fucked in the second time through the opening sequence I stopped caring. Such a stupid decision that really took me out of an already mediocre campaign.
 
The whole endgame felt like wasted potential. There's a lot of pretty cool stuff going on, but it's presented so poorly, none of it feels impactful.

I was going to be defending the story that everyone was criticizing, because through out the whole game it was pretty decent, Nothing horrible, and it kept me interested, then they pulled this shit with the ending? I would really like to know who decided that ending was good enough, because they are an idiot. 

I really can't imagine how a sequel would be possible. Of course, presumably many Helghast survived the planet-wide explosion, maybe because they were underground or in bunkers, since we see two soldiers walking around after the credits. 
 
It's just that, the Helghast have been weakened so much that it's ridiculous for them to be a threat again in less than a decade unless magic starts to get involved.

Wait, so you blow up their entire planet? 
 
And you're supposed to be the good guys??

" Wait, so you blow up their entire planet? 
 
And you're supposed to be the good guys?? "

" Wait, so you blow up their entire planet? 
 
And you're supposed to be the good guys?? "

How much money do these guys get paid? It just seemed like they ran out of ideas on how to conclude a trilogy in a satisfying manner and just said, bugger it, huge explosion, everyone on helghan dead, the end... It makes me so angry when i found the source material really interesting but the story writers don't have a clue what they're doing.

In all fairness it was about as sudden as the end of KZ2. Rico shoots Visari because he is a dick and you go out on the front steps...credits. KZ3: Rico and you shoot down Stahl's ship with a nuke because he is a dick, you chill out for a sec looking at the glo-worm planet...credits.
I hope Stahl returns because he is a great character in mediocre story, kind of like Radec last game. You want to know more about that guy before you knife him in the face.

" How much money do these guys get paid? It just seemed like they ran out of ideas on how to conclude a trilogy in a satisfying manner and just said, bugger it, huge explosion, everyone on helghan dead, the end... It makes me so angry when i found the source material really interesting but the story writers don't have a clue what they're doing. "

Thinking about it more, I'm almost certain Killzone 4 will be saved for the PS4, to go with the new setting/direction they need. If this generation too much longer, I'm betting on a prequel to the start of the war to tide Guerilla over. For posterity's sake:
 
I'm calling it.

I'm hoping that they end up patching in a new ending.  I'd rather pretend this one never happened.
 
@Donos said:

" Thinking about it more, I'm almost certain Killzone 4 will be saved for the PS4, to go with the new setting/direction they need. If this generation too much longer, I'm betting on a prequel to the start of the war to tide Guerilla over. For posterity's sake:
 
I'm calling it. "

" @Dany said:

" as someone who knows of of the story from bombcasts and reviews. Does rico die? "

I came here for this as well lol. "

" As soon as continuity got fucked in the second time through the opening sequence I stopped caring. Such a stupid decision that really took me out of an already mediocre campaign.  The whole endgame felt like wasted potential. There's a lot of pretty cool stuff going on, but it's presented so poorly, none of it feels impactful. "