They ARE working on the new DLC - Sunset Invasions is just a smallish side job.
That is true and people seem to miss that no matter how much people repeat it. However, I still feel that perhaps their smallish side jobs would be better received and be more meaningful for the game if they expanded on mechanics and things that make sense for the setting and direction of the game.
If they had spare time and the desire to create things like interesting events, plots, little personal diplomacy additions (invite a friend or potential friend, specifically, to . . . I don't know, pick flowers, hunt, whatever, things that can tie into plotting down the line etc.), bits and bobs for portraits of current cultures, ninja turtles, even more music or what have you then that would, at least for me, be something I wouldn't hesitate to buy as an aside to other things bigger DLC. I mean, I already do pick up music packs, coat of arms DLC and such.
I have to say though, and people whom are particulary against how ahistorical this is probably wouldn't agree, but personally I think if they really wanted to add Aztecs then it would be more interesting if it was a bigger, much more fleshed out, not to mention playable addition.
I'm all for creating ways to change the dynamic to make the game play a bit differently, and it being ahistorical isn't a huge deal for me, but then this seems to -- so far -- just be a stack of doom with some new art. It seems underwhelming, although perhaps that's because it also seems so random. I don't know. It could also be because afaik there isn't much aside from this Destructoid article to tell us about this, and it really doesn't say much at all.