I'd say first ability to choose civ name (so my Antiquity Americans just happen to have Legions, or my Modern Romans have Marines)..The devs have confirmed that they are working on updating how we identify with Civs as a high priority. I'd say it's fair to expect/hope for tweaks or updates to Civ switching in the roadmap. I'd expect them to be pretty intertwined unless the devs don't want to spill any beans.
Personally I'm hoping for the ability to transcend your civ a-la Humankind.
with the later possibility of having "out of age civs"... ie you can start with "Antiquity Spain" (which has generic Antiquity Expansionist & Military bonuses)..it unlocks regular Spain with its unique bonuses, and regular Spain is the only unlock for "Modern Spain" (which has generic Modern Expansionist & Military bonuses)
Both of those are important so that someone can say
my "Roman Empire" changed in response to the times (even if they were still Roman)
OR
my "Roman Empire" had a very minimal change in response to the times. (it kept the Cultural Militarism)
OR
my "American Empire" was once called Roman, but the name has changed as our practices have changed
Giving the player that level of control over the identity would improve the identity.
But also hoping for....
Hotseat
per Era victory conditions
Improving Exploration and Modern pacing (and in general)*** they have mentioned working on religion
and honest UI that actually provides information
And Post and in game information/Decorations.... we need a Customizable Palace/Throne Room and a Full Map Replay with Demographics, etc.
If we are going to build something we believe in.... then let us see the Thing we have built when we are done.