This is quite a nice idea - always thought Wonders were just eye candy.
From a modding perspective you would just create a series of super-units and technologies that can only be trained/researched at the wonder. To do this:
1. Create the new super-units. (these could just be copies of standard units with better HP, armor, attack) and add them to the protox.
2. Create the new techs, eg. make all the heroes invincible.
3. Modify the Wonder proto definition to add the relevant <train> and <tech> statements for the above.You could also give the wonder various hand or ranged attacks to liven it up a bit.