REV needs to be worked on, but unless we get someone from the original REV team, noone currently wishes to touch it and from what I understand... I can't blame them, definitely since they all have so much on their respective plates already (in regards to the C2C team).
Next is the problem that most of the 'barbarian' hordes that put pressure on the civilized world have themselves become civilizations, such as the Turks and Mongolians, and these are only the big name players.
Another problem is that a small nation, in vanilla, could keep up in the tech race by virtue of a scaling research cost that larger nations had problems keeping up with due to the inherent nature of keeping a amazing infrastructure that a smaller nation could easily manage. Not saying large nations meant losing, however, I can have a huge settled hinterland and keep on par with tech in the tech race.
So, in regards to the 1st thing, not much can be done.
As for the 2nd and 3rd points, maybe something can be tweaked somewhere that the barbarians ignore tiny nations and rampage the big ones. Furthermore, the Great Wall should not be all encompassing and should carry the risk of a barbarian horde getting through as was historical. "Can't go through the wall? Go around it!"... to sum that up, the 'Great Wall is OP'.
On my 3rd point, the tech scaling is just something hard to touch upon anyway, and a LOT of work is being done with it... I don't know what else to say other than it 'should scale better', so that smaller nations are not left to the 'dustbin of history' just because they have less cities that someone else. Thy should, in essence be competitive until they are actually crushed.
I would rather not penalize a player for playing well I, however, would rather the solution be something that adds an additional challenge that is hard to circumvent simply because that nation is the best. Increase the pressure, essentially, with over expanded nations... should lead to it crumbling. Or so the theory goes... If managed well though, should slow down the player's growth.
In summary,
-REV isn't being touched.
-Great Wall is OP, seriously, no way for barbarians to enter ANYWHERE... The Chinese definitely did not have THAT sort of protection.
-Small nations need to be just as competitive towards the big nations. Germany was competitive and could go toe to toe with Russia... as it stands Russia laughs at the barbaric Germans because they don't know how to make fire whereas they are smelting their first iron ingot.
- Something needs to keep the pressure on the largest nation, REV and Barbarian hordes should be kept relevant to gameplay as large nations usually had internal problems galore with various cultures and religions causing troubles.
I find that early ship movements too fast, and modern ship movements I can't comment because I rarely get to that point in the game. Maybe oceans should occur a increase movement cost, similar to forests/hills to land units without the appropriate promotions.
OR... an ocean improvement.
Supply lines/Trade routes.
Easy to build, provides movement bonus, acts like a road in terms of trade connections.
Pillage wise, very wealthy if it has active trade over it.
Edit: In regards to that suggestion, this means I will NOT say as England automatically have a sea trade connection to China UNLESS I have a trade route that stretches ALL THE WAY to China. Forts in this regard should allow trade to pass through as well, at a certain tech level so something like the Suez is viable, or creating the Dardenelles pass to create a connection to the Black Sea.
This also means that the trade routes need protection, stipulating a naval presence along the entirety of the route.
I generally got this idea from the trading system being developed for Europa Universalis IV.
Edit 2: The new scaling city limits might help, playing a game with them now to see how well...
Edit 3: In regards to Edit 1, connects resources on islands, like the island with stone near Athens (Greek starting point), or I believe copper on Cyprus and Wine on Crete.