Yeah, they definitely had the campaign and horde DLC ready long before they released it. Same with the characters and most weapon skins.
I doubt they have a bunch of maps done now, though. Like I said, it's probably up to MS at this point if they are going to release more DLC. It didn't sound like they have any plans for more DLC as of now, if we're going by that interview.
If they release another map pack, I'd say that it will be a classic map pack, because that would be much easier and cheaper to make than brand new maps.
I agree that Guardian should have been there from day 1. Whoever thought that combining Guardian and Submission would be a good idea, should be fired. CTL is such a shitty mode compared to Guardian.