Well, this is unexpected and surely positive news! Didn't think Firaxis would have it in them to make a change like this. To me, what they definitely need to accomplish is this:
1.) You can play any Civ throughout the Ages, including Modern Civs like America! That would also be especially important for TSL Maps!
2.) You have at the least the option that Leaders and Civs match. So no more playing against Catherine of Greece, there must be an option to prevent this.
3.) There must also be an option for your opponent's Civ to stay the same throughout all ages, not just yours!
I think all these three requirements should be fairly easy to accomplish. I also expect some further stuff, like some new unique units even if you only play one Civ through the ages, but that is actually not that important to me. I haven't bought the game yet, but obviously there are lots of other issues besides the Civ Switching that need to be fixed. I also want Firaxis to focus on them, too. I don't think Civ 7 will ever become my favorite Civ game (I'm not a huge fan of Ed Beach's approach for this game in general, and I'm not a big Civ 6 fan, either), but at least now, I do see the possibility that I could buy this game eventually, once the most important issues are fixed!