Asia (probably). If I remember correctly, in the last patch, there was a file where it was written about Vietnam and Kubilai Khan. We supposed it was pack 5 because it's the only one with one civ and two rulers (meaning a ruler for another preexisting civ), and Kubilai Khan has been supposed to be a dual leader for both China and Mongolia. We already kind of knew it was Kubilai Khan because pack #5 needed Rise and Fall, not Gathering Storm; the only difference between the two is the Civs (because all mechanics from R&F have been added in GS: loyalty, governors...). This must mean the alt leader would be for a civ from GS. Mapuches/Scotland/Cree/Georgia are great but not the kind of civs that would need a second leader before, says, China, Egypt, Rome, Germany or India (I refuse to see Gandhi as a true alt leader). An alt leader for Netherlands, Korea or Zulus would be neat (and asked) but 1) Netherlands are a maritime trade civ and another leader would not bring new experience 2) same for Zulu and warfare 3) same for Korea and science. The only choice is Mongolia.
Now we go further into the speculation thread but there was a LUA/CUA that has been in some files for a while being "Gain a free Eureka and Inspiration each time you finish a Trade Route". It would be perfect for Kubilai Khan, first for historical reasons (he used the Silk Road as a way to improve cultural/scientific exchange between Orient and Occident) and secondly and most importantly for me, it would synergize incredibly well with both Mongolia and China CUA.
China: gain +10% from eurekas and inspirations, meaning that if you gain some free they would be free 50% to a tech and civic each time you end a trade route.
Mongolia: Örtöö is already a CUA pretty much entirely focused around trade routes (trade post at the beginning rather than the end, bigger diplomatic visibility from trade posts), so having on top of that another LUA revolving around trade routes would be the cherry on the cake.
I really hope I'm right about this, but it screams to be in the game. It would be an incredibly simple yet incredibly effective and fun mechanics without it needing to be overly complicated. A perfect design, if you want my opinion.