Japanese, Greeks and Chinese all three they really need a boost, first two I find useless, Chinese aren't that bad up to a certain age, the +1 worker is always a +1 worker... Their problem is that at some point, they simply can't compete with the other armies, but they are worth considering, I started with them then moved to Roman, but I have missed the +1 CItizen.
Then it's Romans, I feel it's in the middle, not in the bottom tier, but isn't a top anymore. They really need a boost like German had, from +10% bonus to +15%, a boost to their UU (useless curently), or a decent second bonus, like all other top Nations like Germany Korea or UK have.
Germany, I think the best nation currently in raw power, for war attacks and 5 star attacks, +15% instead of Romans bonus, and they have a second nice hability, far better than Romans currently imo.
The silver and bronze seats, for Korea and Great Britain, both UU really amazing, Korea is better for war (+1 tactic), but +15% loot British make they superior imo.
And the kings of our time, in my opinion, once you have reached at least Enlighment age, the French, La Republique. lol . A very balanced nation (good UU, nice hability +6 ally gate, makes them good for defense in wars, and the reduced training time, ok not important for war, but important if you need more resources, or if you need medals, two conditions at least one of which any player is going to need at some point... If you are a hard core Dominator, lol, and play hours and hours, this is probably your option. But still, I think they aren't that imba against the other top nations like Germany British and Korea, so I think game is currently ok.
I will only change that bottom ones, Romans as I said, and the other three need offensive army boost for the late game, if they want to compete against others.
I don't really know of course, but could be things like: Activating a second general is free in oil, for example, +1 air capacity (although that seems to be reserved to future Americans...), mercs cheaper (decent for Chinese for example, could read: "+1 Mercs capacity and they cost 1 less of every product but Diamonds), something related with Kamikazes for Japanese, etc.