While that is technically possible, there are a few constraints and obstacles for us game developers:
- Generally we need to prioritize the work that’s in our contract (like the new civs) and it’s rare that devs are given “free paid time” where they can fix/work on whatever they want
- Balance-wise this is a rather risky enterprise as the older, European civs have an established meta and balance that matured over a decade. These highly perfected playing systems are and react very sensitively to any kind of profound design change. So while it’s not completely impossible (see Ottoman Nizams or Aztec Eagle Scout) it does require notable balance efforts and discussion which are usually required elsewhere.
- In terms of civ design spreading new mechanics to other civs always bears the risk of eroding the uniqueness of the new civs. Now that does apply more to certain mechanics than others. For example, a single ported/repurposed unit or card wouldn’t turn the British into lets say USA 2.0, but applying their federal card mechanic 1:1 to all Euro civs would deprive the US one of their most iconic features.
What is possible and we’re also trying to accomplish in balance updates is to open up new and more possibilities for the older civs. 