I like this idea, unique civ things are good for differentiation and interest

That's because building and holding a Wonder is one of the available win conditions. That's balanced by build time, more than a cost. 

But thats because most heros are recruited in the fort. If they were recruited in the wonder, everyone would build wonders. Well, Fort+Will to Fight is 1500/1800/2100/1500 while a Wonder+Glorious Expansion is 2000/4000/2000/1500, so its really only more expensive regarding wood (and a bit food I guess). The real problem is this; The wonder takes way too long to be built. 1000s?? More than twice the time of a fort (450s)?? Insane.

that's what i was suggesting, either more damage or cheaper units in some way

Yes.  20% more population, plus a really good unique technology for every civ. I like Wonders as they are, as I don't build them anyway

This is why I think any change should be only done to the inf spear modifier. Against cav I think this is just a such obvious solution.  Almost everyone agrees that champ cav are too strong against cs spear inf. Any other change would mess with other balance. It’s targeted and gets what everyone thinks should change. 

will to fight is easier to get as most players build a fort in a team game, and not to mention the amount of resources and time it takes to actually build a wonder you're better off spending those resources and time on making champions as doing all that for a wonder just to get an extra 20 pop bonus is not worth it