Pleased that a fascinating historical figure made it in (assuming we're talking about James I and VI, not James II

), but sorely disappointed that the alt leader went to civs that really didn't need it.
I think Scotland's a serious consideration as a civ that could use an alternate leader, but James VI wouldn't be my first choice - rather I'd expect Calgacus (fictitious as he probably was) or someone else who could be chosen as a representation of "Celtic" Scotland to satisfy players who still feel that "The Celts" are (a) missing and (b) desirable.
For all of Kublai's fanbase, is there any real "need" for a new Mongol or Chinese leader? Qin makes sense for China and Genghis is obviously characteristic for the Mongols, and I don't think either civ is bad.
Has there ever been a case in any Civ game where two civilizations had represented almost the same geographical area?
I'm just curious because some here are discounting Siam and Vietnam as upcoming new civs due to their proximity with the Khmer.
Every game that's had Byzantium alongside the Ottomans and Greece, as well as those that have had Assyria or Sumer alongside Babylon. EDIT: And, much as I try to forget, there's the ridiculous HRE civ in Civ IV, which obviously exhibits essentially complete overlap with Germany.
Siam is out for reasons other than proximity, and was explicitly described as a replacement for the Khmer if I recall correctly in Civ V. The Civ V incarnation (properly called Sukothai) was a civ that emerged as a successor to the Khmer Empire in the same broad time period, and with a very similar culture, architecture and indeed love of elephant UUs.
Both Khmer and Civ VI's version of Indonesia are medieval civs - there's no reason to add a third medieval SE Asian civ that would share the same broad architecture set. I say this as a lover of SE Asian history and, indeed, the medieval period was the heyday for Southeast Asian civs - but it's a comparatively small geographic area in which to insert three medieval Indian-derived cultures (and no others).
Vietnam is much more likely - it has more time periods to choose from (Thailand does indeed have later periods they can use, including the era when it was actually known to outsiders as Siam, but that wouldn't be the civ people are presumably asking for as Siam - i.e. the Civ V version), with a relatively modern period seeming favoured by a lot of people requesting it (although, conversely, they could go with Classical era Vietnam, the era of the Trung Sisters), and with a corresponding variety of approaches that can be taken with it. Vietnam was rarely as prominent as the other Southeast Asian civs were in their heyday, but it has a much older, broadly continuous history as a unitary state (or at least a series of states within the country's modern borders) than any of the others, with the Champa and Khmer periods being the main interruption.
It also represents the major Chinese-influenced culture in Southeast Asia, while the others have affinities with India.