I dont know if this was mentioned again but imo we should make a list from the most op civ to the less and try to change some minor things to the most underused civs like brits/turks/koreans etc..big changes or changes affecting alrdy strong civs like the one i saw for chinese would be dangerous.
ps. about wonders they could allow player a unique economy tech, sth like next step of hand cart etc maybe
the_Dragon has a good point here in my opinion. I do not relate wonders to military matters, its a pure civic achievement.
Because wonders are often build at a point the bottom of your stone or gold pile comes in sight, it would be a way to extend your resource input and the game. Like having a wonder could reward you some gold (and stone?) like owning a relic? (but then bigger, like a scaled up post-imp version of this)
Another way: have a wonder enabling you higher pop limit, like +25 or +50 for every wonder (maybe depending on civ, im sure you are able to balance this things out)
IMO, the Keep, upgraded from Guard Tower, is not powerful enough to be used in imperial age, especially when stones are expensive to buy. Maybe a Balllista Tower like AGE I would do the job. idk...