The conquests campaign in C3C, created long ago by Ed Beach for that Civ 3 expansion, in my eyes seems to be the basic idea of this "civ-switching" in Civ 7. What Ed Beach achieved long ago, at least now should be possible, too (and many features now should work much better). So I think it could be interesting to have a look into the construction of such a
C3C campaign.
In the C3C campaign 9 complete different scenarios with complete different civs and complete different leaders are connected to a campaign. The player receives a score for each of those scenarios and a total score for the campaign can be built.
In Civ 7 we can have three eras, each of them is equivalent to one of those Civ 3 Conquests. As in C3C many years ago, it was yet possible to connect much more than only three scenarios, of course this should be possible with Civ 7, too. That´s why in the long time I expect much more than only 3 eras for Civ 7. The mechanism for this is known since Civ 3.
This also allows to have in Civ 7 much more civs, that can evolve in their next "version" as now it is shown with the "three Indias" or the "three Chinas" which are in the current Civ 7 discussions.
When trying, as a Civ 3 modder, "to build a Civ 7-like mod", I have as one of the problems, that I cannot transform the campaign points from the score of ending one scenario as an additional benefit to the next scenario - here Firaxis had to introduce an additional mechanism. It is also not possible in C3C to do a "reset" when entering the next era in a game. In the C3C campaign a new conquests scenario must be loaded. The new loaded scenario can have the same civs (may be with new names and new leaders) and the same map and aditional may be new named cities on the map, a new techtree and new units without a place for units of the former era - but the only "legacy" you can carry to the new scenario in the C3C campaign is the score.
On the other side with the C3X mod it is yet possible in C3C to give the leaders and civs different names in the four different eras of C3C. So I am very confident, that Civ 7 without bigger problems - and continuing in its current concept - can be able to give us civers the "three-civs-solution" for many of the upcoming civs, too. What is possible for Civ 3 should be possible for Civ 7, too.