I would hold my horses bit more when it comes towards getting new civs. Even a third expansion or any DLC after GS is far from confirmed, so hoping for DLC after the third expansion is pretty much speculation without any base.
Speculation yes, but not baseless. For a number of reasons, 2K has every incentive to invest in long term support for their only tentpole; this business model is the direction every developer is moving and VI was clearly designed to support it. Unless and until they find other sources of revenue, VI needs to sustain them. And the cost of moving onto VII--the loss of so much aesthetic investment coupled with another few years of players waiting to make the plunge--is daunting. Right now they get regular incremental income as well as are devleloping as a perennial celebration among the gaming community. It would be hard to stop.
Furthermore, their current model displays meticulous calculation to sell. 8 civs, four returning, four new or fixed. Three female leaders: just enough representation without overwhelming the male playerbase. One classic domination civ to sell the thing to veterans. One quirky modern sports based civ to sell the thing to casuals.
This model can be scaled to any size DLC. A DLC pack with one highly requested classic would sell regardless of what other civ(s) are attached to it. Hell we could argue that Mongolia and the Ottomans single handedly sell the current expacks. And the same could be said of any leaders expack which included George Washington or Napoleon. Doesn't matter if it's two or twenty, doesn't matter how obscure the leaders are.
So really, under the current business model, the devs appear to feel like they can keep developing content as long as they have major draws up their sleeve. Portugal will likely be that civ that sells a third expack. And after that, DLC packs of Ethiopia + X, Morocco + X, Denmark + X, Maya + X, whatever they missed in the third expack, would still be fairly safe investments. Especially if they were a little extra and included new scenarios or alt leaders to sweeten the deal. Hell, I would even argue certain countries with high playerbases would also be safe investments and could sell an expack composed of completely new civs: Mexico, Ireland, and Italy in particular.
Regardless, as impressive as VI already is, I get the overwhelming impression it has at least eight more civs planned conservatively, and optimistically will be milking the game even further beyond that.