Is there any particular reason why you think the Timurids are considered a priority? I think that Scythia covers that area pretty well so I'm not sure there is a need for another nomadic civ from Central Asia. At least that's what I think. I'd put the Navajo/Apache there instead. Besides Timur is a Great General right now who replaced Genghis Khan in R&F so I don't know if they want to replace him again.
It's a combination of several reasons.
1. Players really want a Gurkhani civ. It consistently is in the top of request lists, usually the Mughals but the Timurids get a fair amount of attention as well.
2. In a game that seems to be wanting to fill large culturally related gaps on the map, we don't really have a civ representing the "stans." Persia doesn't really cover that, and the fact that we have Kabul representing the Durrani means there could still be some "larger" empire they want to fill that region. And, coincidentally, we still don't have a Samarkand CS.
3. The Mughals seem more complicated to implement. Yes, they have a convenient female leader in Nur Jahan, but they also have a lion icon that could be easily confused with Ethiopia. Also, the way India was implemented begs that they be blobbed into India, especially with the stepwell as a UI. A counterexample for why a separate Mughal civ would be awkward is if we got a third India leader from the Chola; why would the Chola be "India" but not the Mughals?
4. The Timurids kind of function as a spiritual predecessor for the Mughals in the same way Phoenicia vicariously represents Carthage or the Maori kind of represent the rest of Polynesia. The Timurids pull a lot more conceptual weight because they occupy a distinct region while simultaneously nodding to other important civs in the region.
5. Tamerlane would be, on average, easier to design than an Akbar. He's practically a cartoon, like the anti-Bolivar. Not a huge factor but still there.
On balance I just see the Timurids checking a lot more boxes; they could still have a strong cultural bent like the Mughals would, maybe even with some nods to the Mughals, while still fleshing out the game more thoroughly.
EDIT: Yes, we could have an Afghani empire or the Sikh Confederacy, but neither really "covers" the region and feels more like a CS, hence Kabul. Plus they just aren't as resonant and easily designed. I think for more finely divided cultural regions of the world the devs prefer to pick a broad-reaching historical empire rather than try to go through the pains of sensitively representing each distinct polity.
It's hard to say which Civs will but there is a possibility that they had an idea of the Civs that would eventually make it so the later city-states that replaced a previous one might not get in.
Seoul into Babylon (Even got a ramped up new suzerain bonus)
Amsterdam into Antioch-Byzantium (Then again Byzantines might not even need Antioch)
Stockholm into Bologna-Italy
Toronto into Mexico City-Mexico (Doubt Mexico is coming anyway)
Lisbon is from the base game so I think it does make Portugal more likely. I hope my theory is wrong though because I want Italy.
This too, I forgot about the new CS. I'm not sure exactly what Antioch is intended to represent, and we could always still have some softer versions of Italy, but I do think that Babylon is pretty unlikely.