I am currently playing 2 practice games, so I can give my thoughts on them. Keep in mind that even though I generally play on Deity, I am bad

and occasionally reload. And I have never won SV before t200.
Game 1: Arabia, difficulty 6.
I got double DoW'ed by Isabella and Gajah before I had even settled my second city, somewhere in the t30's. Oddly enough, Isabella never moved her units towards my borders, and Gajah harassed me with a couple of archers. I think I then had to reload around t130 to send Napoleon after Celts. Now he DoW'ed me again, but the position is easily defendable because I have units now, and my religion alone is giving me 83 gpt (tithe), so I just buy buildings with merchant dynasties. Speaking of religion, take a look at the screenshots: tithe and evangelism are doing lots of work. With evangelism, my cheap missionaries battle out Theodora's prophets -- but that's something you could do in the base game. Happiness is also not an issue due to my massive religion spread (and I am not yet done).
Policies: Piety -> Commerce. Pocatello is the leader, and he just entered Industrial on t160, which would the case for vanilla Deity, I think. However, I stole like 5 techs from Bouddica, and that's keeping me afloat. I could potentially go for bombers + tanks domination, provided there is enough oil, but the map is large, so I don't know if I want to spend like 10 hours on it. I will also have to steal a lot of techs. In general, the start wasn't too difficult, but the spawn location is pretty crappy for anything other than religion, and I don't think that science or culture are possible at all.
Game 2: France, difficulty 5. Tried a "wide" Liberty game, but had to replay because of some things I did not know about the mod.
Much easier game, and I am on par with the AI in terms of technology. The plan was to expand to 8-9 cities, but then Pocatello dropped a city in my face, and I decided to simply conquer it. 4 CB's and a spear took defense 8 city no problem. Then I went ahead and burned a Mayan city, which made him give me one other city in white peace. I sold that city to Pocatello. When Alex and Shoshone double-DoW'ed me, I reloaded and bribed. Eventually Alex still DoW'ed me, but thankfully I had Chivalry by then and was able to kill his hoplites with pikemen. As you can see, terrain is poor on food, and I am spamming chateaux, but it is difficulty 5, so I will be fine.
Policies: Liberty -> 4 Piety -> Aesthetics.
Some thoughts:
1. AI is still too aggressive. While it might seem like a good thing and challenge the player, it also hurts other AI's who could be potential runaways. For instance, the Mayans could be running away with science or culture, but because Alex and Pocatello have such strong early game, they will just wipe him out, which makes it easier for the human player to go for CV or SV, as long as you defend against those two. Of course, they can also start running away with culture themselves. But I think that overall AI's are fighting each other all the time, and that makes the game a little less enjoyable.
2. Because the AI's are fighting and always angry, it makes friendship very difficult. In the Arabia game, I was at war with all my neighbors, and only Isabella became friendly after t150 or something when she adopted my religion. Otherwise, you can't really send trade routes out (hurts liberty game or Morocco) -- and they don't really affect diplomacy, it seems. The AI's always covet your lands and think you are expanding aggressively, even when you have 2 cities. In the France game, everybody hates me. Sure, I took a city and razed one, but that was 70 turns ago, but I am still being denounced for warmongering. I stopped expanding nearly 100 turns ago, and the AI's still think I am building cities too aggressively. All in all, it is a very hostile environment.
3. On units. While I like that melee units are much better now and matter more, I almost feel like now they are so good they make ranged units pretty useless. Ranged units do very little damage, and given the costs, you are better off just spam-building melee units and ramming them into other melee units and AI cities. I think that in the current situation, either ranged units should be cheaper, or melee units should be more expensive to build/buy. In the France game, Alex's hoplites together with Honor are so good that they are basically pikemen. On higher difficulties, he will have them earlier, while the human player has a long way to Chivalry. Given that pikemen are now at Chivalry, I feel like now you must beeline Chivalry in most games because otherwise you won't be able to defend yourself against AI or go for any conquest because swordsmen are pretty weak in comparison and ranged units don't do anything.
I haven't played demigod or deity, but I think you are approaching the fine line between challenging and impossible.
