Killzone is irrelevant compared to Halo. Personally I think it's a joke of a game compared to any of the Halo games from a gameplay/campaign/story standpoint, but regardless it doesn't sell gangbusters. I couldn't find good numbers for the most recent games (which is telling they don't trumpet them) and the site-that-must-not-be-named gave the last PS3 game only around 1.6 mil sold after five months, the worst-selling of the franchise according to them.
Sure, it's a "big" franchise, but it's not in the realm of games like Uncharted or Halo, let alone GTA or COD. Frankly I don't think they have much money to lose by giving away maps for free, especially if they churned out extra during development so it's not a major time sink.
I think aggressive bundling of DLC or DLC going free after X amount of time, with goodies sweetening the deal for those who pay, seem like the best options. I'd rather have that than microtransactions for cosmetic stuff--that should be free and part of the progression system.
In terms of monetization and DLC, however--Going along with my palettes idea, what if you could buy extra tilesets (Halo 3 Forge-style Stonerunner, for instance), effects (Fog! Snow!) and objects after release? If they were playable for everyone but you could only Forge maps using that tileset if you paid, that would seem like something that people would enjoy if it meant more post-release additions to the Forge palette (I doubt we're going to get another Forge Island in the future, and really we shouldn't if they build Forge properly with a large canvas map at launch for the next game.)
