The problem with the all in one mod (and with a theoretical all-nations-playable mod) is that balance is just completely broken. Balance is already not great in Dominions, but that mod destroys it to the point on uselessness in most cases (mostly when you start looking at things like how capitals generate different amounts of gems in different ages and how mages are overall much more powerful in EA than LA, among other things). Also it tends to lead to games that are unfeasible because if you want all of the nations available you either end up with 2 R'lyehs, 2-3 Ermors, and a few other nations if you have a small group (where I guess balance is decently preserved, but that's... not a very fun game for many people) or you have a game with 10+ players, which rapidly decays into staled turns and missing players.
It's possible, but you would have to build the nations from the ground up with that in mind. I guess that makes it a job better suited for the developers, but it also means that it's more difficult to give nations a unique identity in different ages. It's... honestly not worth the effort.
All that said, I really like the idea of alternate timeline version of nations. LA Ermor that beat the curse, LA Caelum with their divine influence intact, maybe a MA or LA R'lyeh with more aboleths (I was always a little sad that the giant fish pretty much disappeared when the squidfaces showed up).