This is probably severe optimism at work, but the more I think about it, the more I think you could be right, and Gran Colombia is taking the place of a second Northern Native American Civ. We have never before had an expansion or collection of DLC that has added more than two Civs from the Americas at the same time. No doubt the Cree alone don't offer sufficient representation for the myriad Native American cultures of North America, but with only 5 slots left there are bound to still be gaps.
So this is my second (slightly more detailed and incredibly optimistic Italy-inclusive) take:
Pack 2 - ETHIOPIA
We already know about secret societies, so I'm betting that the new District and buildings are espionage-related. Secret Society Mode is R&F/GS-only, so it must be working with loyalty, eras or governors somehow.
Pack 3 - BYZANTIUM and ITALY
This is adding a new R&F and GS-only game mode, plus new Wonders and a new map. The reason I've grouped these two together is they're both Mediterranean Civs, so the new map could be a Mediterranean map (also, if the Gran Colombia & Maya pack is any indication, they might quite like to pair a returning Civ with a new one). There are plenty of good candidates for Italian-themed Wonders, but I suppose there doesn't have to be a connection. Game mode: something to do with religion might go nicely with these two, or possibly culture/tourism? It would have to involve a R&F mechanic somehow so my guess is loyalty or maybe Ages.
Pack 4 - PORTUGAL
Another new game mode - my guess is trade or possibly exploration/colonialism-themed. Portugal could be regionally grouped with Byzantium or Italy, but the city-state thing still makes me think it's coming here. It's also theoretically possible they could put Italy here if they're adding lots of city-states and Great People, if an Italian Civ has some kind of synergy with those.
Pack 5 - VIETNAM and alt leader for MONGOLIA or KOREA
As discussed, the new leader is likely for an R&F Civ. I think it's a strange choice over e.g. China or Egypt, but I can't rationalise the requirement for R&F any other way. I'm still leaning more towards Mongolia but Korea is also a distinct possibility. I suspect this is also where the plague/disease game mode will go, since this is where we're getting the second new District & buildings, which surely must be health-related, assuming the one that comes with Ethiopia is espionage-related.
Pack 6 - BABYLON
If we are not getting another Native American Civ, I'm now thinking Babylon will go here. We could have an ancient Mesopotamia map. No idea what the final game mode could be. They might do something else a bit wacky like the Apocalypse mode. Leaving a big, much-requested Civ like Babylon until last would ensure people are still interested in picking up the final DLC too.
*edited: got the game mode exclusivity the wrong way around*
**edited again: MORE than 2 American Civs at the same time**