Assyria.
I've played it.
The Assyria in this game is technically the Middle Assyrian Empire, 1392-934 BC. This is not the enormous Neo-Assyrian Empire that died in 612-609 BC, nor the cruder Old Assyrian Empire that preceded it. The Assyrians were noted for their efficient army and well-ordered (though often harsh) society. These characteristics helped them survive the Bronze Age Collapse that brought down most other Eastern Mediterranean and Middle-Eastern Bronze Age societies between 1200 BC and 900 BC - including Mycenae, the Httites, and even Egypt. You start before the Fall, you reign through the tumult (you're mostly on the edges but will have to hold off Sea People in the Levant and Anatolia) and your UHV needs to be achieved by 934 (better 950) BC.
UHV 1 has to be Conquest.
King of the Four Corners of the World, an Akkadian-derived title referring to northern Subartu (Assyria), western Martu (Levant), eastern Elam (Iran) and southern Sumer. RFC GW already has defined regions encompassing these, although it would be better to define separate Subartu and Sumer regions in order to force the player to both control Sumeria and also settle a separate northern city (I have Nineveh in mind), rather than catch-all "Mesopotamia". Middle Assyria rarely used the title, but these were always areas their kings wanted to control.
Assyria spawns in 1396 BC, and a reasonable deadline for UHV 1 would be "by 1250 BC" or "in 1250 BC" (right in the middle of Shalmaneser's reign). There is no challenging historical date we can pick, but using the initial army and a little tactical thinking it should be possible (on Monarchy) to conquer three Corners and settle the fourth, in that time. In fact 1300 BC is possible if Elam has collapsed or was conquered, whereupon every city in Elam flips.This means that the player may only need to conquer southern Mesopotamia and the Levant and train one Settler. However, the UHV has to recognise that if Elam is alive then there may be a long war in the east, plus on higher levels of difficulty you will need to build replacement units because losses will be higher.
Edit: Actually a 2-stage UHV 1 seems more challenging: you need to hold the Corners
by 1250 BC
and in 1150 BC. That should be too early for anyone to have finished UHV 2 (if not, UHV 2 is the one that needs tweaking), so the "in" clause shouldn't cause an issue. This would stop people gaming UHV 2 by dumping regions on their neighbours to speed up science.
UHV 2 will be Science. Assyria is well equipped to do research, if Assyria, Babylon and Ur are properly developed. From the earlier civs, only Minoa has a major technological race and the AI generally fails to progress far with it. Assyria is one civ that even the AI seems to do well with, technologically! The UHV should require the player to stretch a little, and should punish players that have too many cities. The choice is between "first to discover ..." and "discover ... by 950 BC".
Record Keeping (need
Herbalism,
Alphabet,
Writing,
Code of Laws),
Tyranny (same as Record Keeping) and
Construction (need Masonry,
Alphabet and Mathematics) seem good, but need playtesting. I'd like to include a military goal but didn't want to make it too hard. If we need to give the player a proper workout,
Military Science (
Herbalism, Polytheism, Mythology, Storytelling,
Alphabet,
Writing,
Code of Laws, Tyranny, Citizenship, Philosophy) could replace Tyranny in the list (in fact you need Tyranny for Military Science). We need it tough enough so a player who beelines it might get there but would need to skip useful techs along the way. I favour "discover ... by 950 BC"
.
UHV 3 will be ... something else. Something difficult. Culture is in the air for Assyria, so no arbitrary Culture goal. Conquest is easy for Assyria; even demanding expansion to or beyond the borders of the later Neo-Assyrian Emprire is within reach - although Stability may bring it down. But RFC GW seems more forgiving of poor Stability than vanilla RFC. Besides, UHV 1 was miitary. Either way, if you want Conquest or Domination, play Conquest or Domination. It needs to be something Assyria would struggle with. Have Open Borders with every other surviving civ in 950 BC, say, with at least Cautious (all two or three of them ... yeah). Problem is, I kinda hate "in" UHVs - it means you must play till that date arrives; you have litte scope to demonstrate excellence. Hence the existence of "virtual UHVs" where "I had it in 800 BC but had to wait til the UHV came around in 500 BC". So maybe change it to "before" 950 BC. If in any year you have Cautious OB with everyone, tick!
The
Unique Power needs to change. Assyria enslaves a ridiculous number of barb units! After a while I started killing most of the unwanted Workers. So, first, barb units need to be immune to the Assyrian power (you can kill 'em but you can't tame 'em), and second, instead of enslaving them as Workers, they should be recruited to the Assyrian army as whatever unit type they are! They would come in as raw recruits without any XPs or promotions, but over time you would be able recruit e.g. Bowmen, War Elephants, etc, to your ranks. This is similar to what already happens when you destroy a Worker - it is recreated as a Worker for you; but with your UP a military unit would still be a military unit. Oh yes - if the unit is hosting a Great General, the GG would not be lost but would stay with the unit.
Edited to add (26/2/20): Got a rough start bench testing my proposed UHVs today. Babylon and Elam had both planted useless cities in my core. Razed them, of course. The double razing penaltly spiked my Stability and the diversion slowed me down, and I captured Aleppo to complete the Corners for UHV 1 in 1252 BC - literally at the last moment. All seemed to be going well until I collapsed in 1188 BC.

So, hmm, maybe I just won't test-play starts where I have to raze enemy cities built in my core. And maybe I'll allow a little more time to conquer the Corners, because, sure you
can get 'em all by 1250 BC, I've proved it; but what's the point if the dead weight of all those unhappy cities collapses you? 1200 BC should be soon enough.