Well because looking at the rise and fall roster if civs there isn't one that i believe is on the first spot for getting a second leader.
I mean in rise and fall we had: Cree, Netherlands, Mongolia, Georgia, Scotland, Mapuche, Zulu and Korea.
None of them and i repeat none of them for me need a second leader. Not a single one.
Because i doesn't make sense to give a leader to Scotland or korea or Mapuche when other long lastin major civilization won't have anything like egypt or China.
The only thing i find plausible with this theory is only Kublai Khan, because it would be a leader of both China and Mongolia and i would be fine with this. Because it fits in quite nicely with the other second leader choices, greece for the athens/sparta duality, india because is such a huge and long lasting civilization that deserved another leader, and england/ france since they are major players in Europe.
Other option are just too out in the blue imo.
And another thing, firaxis has clearly stated that they are first of all concentrating on new playstyles rather than the civs themselves, so it is more probable they have found a unique playstyle using some rise and fall mechanics (does gathering storm really have all the rise and fall mechanics? So if someone wanted to buy rise and fall or gathering storm now they would be getting r&f only for the additional civs? It's a bit strange but i don't really know the answer so it's probably just my ignorance) and then found a civ to connect it with and not the other way around.
That's my point of view though, i will stand corrected when that pack is revealed. Who knows.
Yes, Gathering Storm really does include all of the mechanics introduced in Rise & Fall. There's no reason for the teaser to specify "Requires Rise & Fall" unless the alternate leader was for a R&F civilization. So, whether you like it or not, one of those "unworthy" civilizations is getting a second leader.