Lord Lakely
Idea Fountain
Let's be honest. Most Civs will have been chosen for their diversity and historical relevance and the (pseudo)-historical decision of who can be developed into what is just a question of what is the least nonsensical geographically.
Like a very loaded game of Connecting the Dots.
Ideally, we'll end up with enough Civs in every Era so that we have a logical transition for everyone.
but if we want to guess who can turn into what, perhaps we should work backwards, with the Age 2/3 options first?
Byzantium via Rome or Greece
Kievan Rus via Norse
France via the Normans
Germany via the HRE/Teutons via the Romans or Goths.

Ideally, we'll end up with enough Civs in every Era so that we have a logical transition for everyone.
but if we want to guess who can turn into what, perhaps we should work backwards, with the Age 2/3 options first?
Byzantium via Rome or Greece
Kievan Rus via Norse
France via the Normans
Germany via the HRE/Teutons via the Romans or Goths.