It seems fairly clear leaders have been decoupled from civs. I'm of the mind i'f always prefer new Civ being released before additional leaders for existing civs. It did get me thinking. How much easier would it be to release a new Civ compared to simply a new leader? In effect, would the time spent by the artists be any less for a new leader?
Well, I've heard that in Civ 5, new leaders took up a majority of development time compared to a whole new civ -- some have said as much as 75% here! Which, I can definitely believe, considering everything that went into Civ 5 leaders (a bit wasteful, in my own opinion). Still what those actual costs really are, and how they affect and compare to the amount of revenue generated by these DLC's is another consideration.
Anyway, personally, I prefer alternate leaders as DLC. They just feel more "optional" than whole new civs (though, I guess making DLC that "you must have" is a good incentive for Firaxis, haha). They could make a whole bunch of them and you just pick and choose which ones you like -- maybe they could come in packs too with scenarios or something.
Though, I wouldn't mind new civs as well (I just prefer new leaders a bit more), as long as they're not classic civilizations in the franchise. I wasn't a fan of that in Civ 5, and felt that they should've just been released as a part of expansion packs. New DLC civs should be more of the ones whose inclusion would be questionable in an official expansion pack versus other more iconic civs.
Also, alternate leaders avoid any "muddying up" of your civilization pool when creating random games. I personally want to play against big iconic civs in my games, like India, China, Spain, etc. I rather not get games filled with civs like the Sentinelese. I'm not a TSL player, nor do I like to play on immensely huge maps jammed with every available civ; I prefer keeping things Standard.