A brand new image has been posted to the Killzone twitter, suggesting that something will be coming "soon." Despite how vague this information is, it’s pretty clear that whatever that something is, it will be big.
We can only speculate at this point in time - despite the announcement on the Twitter page, we have heard very little from within the Guerilla Games studio. No rumors have been spread in relation to Killzone over the last few months, so once again, we’re pretty short for answers.
Some Killzone players are suggesting that the new announcement may be somewhat related to the PS Vita game, named Killzone Mercenary. Mercenary was released in 2013, so it's completely possible that the game could be receiving a timely update.
Alternatively, there could be a completely new Killzone game for the PS Vita. Considering the 2 year gap since the last PS Vita title, it's not unreasonable to think Guerilla Games may completely scrap Mercenary to work on something completely new.
It's completely possible that the new something may have nothing to do with the PS Vita whatsoever. A brand new Killzone game isn't entirely out of the question, although we expect we'll be more likely to see some kind of content patch for the game.
Guerilla Games have already made a lot of changes to the base game of Killzone Shadow Fall, so seeing more changes would not be surprising at this point.
Unfortunately, we can't really make any guesses past what we've shared here. We'll just have to be patient and see what comes in the near future.