Ok, so I tried to imagine civ7 with fully functional TSL Earth (biggest size possible) with forced civ switching and no option to turn it off, ao worst case scenario - how could transitions look?
I use bold to mark civs which are already confirmed or are very likely anyway
ancient NA -> Missisipi -> Lakota, any Native Americans really, 19th century is modern era! You may even make two such evolution lines
Maya ->
Aztec ->
Mexico
Tairona(?) -> Muisca -> Colombia
Caral ->
Inca -> Peru
Achaemenids -> Safavids ->
Iran
Nabateans ->
Abbasids -> Saudi Arabia
ancient Egypt -> Fatimids -> Egypt
Hittites ->
Ottomans -> Turkey
Numidians -> Almoravids -> Morocco
Axum -> Zagwe ->
Ethiopia
Djenne ->
Songhai -> Mali
Bantu -> Swahilli -> Kenya
Athens ->
Byzantium ->
Greece
Rome -> Florence ->
Italy
Celts ->
Normans ->
France/England
Goths? ->
Nordic ->
Sweden/etc
Slavs -> Muscovy ->
Russia
Han -> Song -> PRC
Maurya -> Chola -> India
Funan? -> Medang ->
Indonesia
Jomon ->
Edo Japan -> Meiji Japan
Scythians? ->
Mongolia -> modern Mongolia or uhh Kazakhstan?
Honestly I'd totally accept the latter!
It is worth noting that with the possible exception of four or five ancient civs, Kazakhstan and modern not very impressive state of Mali
all other would be major civilizations being very sensible object of DLC or mods anyway...
Now that I think...
...we could simply combine efforts of modders to create huge TSL projects also containing all necessary transitory modded civs...
All that gets much simpler anyway once if can mod in the ability to retain civs past each era.