While working on my proposed list, I've come up with a loose tier list of various possible connection between civs. Of course, higher tiers are to be prioritized, but sometime some civs will have to make-do with lower tiers because higher tiers do not exist, or because their higher-tier connection is to a civ that would have too many (see: Rome, Han), and it still has to be done while having only a limited number of civs in the game, so not everyone can have top-tier connections.
They are :
S-Tier : the best, but only to be used when the two civilizations can be suitably distinguished and given recognizably different names. Will actually be rare in practice.
Succession. There is a general political and cultural continuity between the two civilizations, or at least alleged continuity that is generally recognized (Ex: Rome-Byzantine, Han-Ming). S-Tier connection.
A-Tier. This will be the most common level of high connection, as direct successors cannot be too common in the game.
Legacy. While there is no political continuity, there is a clearly traceable and significant cultural heritage or evolution from the former to the later. (Roman-Norman)
Claim. The later civ present itself as the successor of the former, and claim their heritage, but the continuity is not evident or contested. (Byzantine-Russian).
Conquest. The later civ conquered the former, resulting in the people of the former civ intermingling with the conquerors and becoming part of a new post-conquest society (Egypt-Abbasid).
B-Tier. Also a pretty common level of good connection.
Colonization. Much as conquest, however with much reduced intermingling. These connections should be made carefully where they involve still-extent colonized people. (Shawnee-American).
Kinship. The later civ belong to the same relatively broad cultural grouping or has a reasonable cultural relation with the former. (
Exchange. There was historical partnersthip, alliance trade and/or influence between the two civilizations or their respective spheres or regions. (Carthage-Songhai, Scot-France).
C-Tier. PReferably there should be at least one higher tier connection, but these will do if nothing else will.
Proximity. There is no particular relation between the two civs, but they existed in relatively good proximity to each other. (Aksum-Swahili).
Speculative. There is no proven historical or generally accepted connection, however there are theories and hypothesis with reasonable support that suggest a potential connection between regions (Tonga-Inca).
Migratory. A significant amount of people of the former civilization (or their kin) settled (or were forcibly move) in the land of the later civilization, and played an important part in its history. (Songhai-America)
D-Tier. These really only should be secondary options.
Referential. The connection is not between these two specific civilization, but represent a higher-tier connection that happened earlier or later in history between their kin (Hawaii-Meiji, alluding to Japanese migration to Hawaii)
Legendary. There is no accepted theory or fact regarding connection, but well known legends, fringe claims or meme refer to it (You know who they are).
F-Tier. These shoudl be avoided wherever possible, but may have to do in some cases ; ideally no more than one and certainly never only those (if a civ only has those connections, it shouldn't be in the game).
Geographic. They were sort of on the same very broad area and otherwise not really connected at all. (Egypt-Songhai).