Quite a few mods add new content along those lines. It depends on what exactly you want. The in-game Mod Browser is generally not a good place to search for content, as many (most?) of the mods in it are nonfunctional due to changes in the core game. These boards are the best place to look.
For instance, if you want new buildings, wonders, units, and techs that go beyond the end of the tech tree, there's my own mod (see .sig). If you want more buildings, units, etc. in the existing eras, there's the community Call to Power mod, Hulfgar's modpack, History in the Making, and Building Made Fun (see the Modpacks forum for these), the NiGHTS mod (see the Projects forum), and if you want something a bit less of a total conversion, the "More Wonders" thread in the Mod Components forum contains a number of specific wonders you can easily add.
And of course, the benchmark by which all "balance" mods are measured is Thalassicus' set of mods (Projects forum). There are a few other "balance" mods, including PlayWithMe (Modpacks) and part of my own mod, but Thal's is by far the most heavily used mod.
Finally, there are "mechanics" mods like the City-State Diplomacy mod or Echoes of Ages, mods that overhaul one specific part of the game, and are generally compatible with most other content mods. And UI-only mods like InfoAddict (Mod Components forum) are very heavily used.
There are others, but this should put you on the right track. Basically, go into the subfora of this area, and look for the active threads; that's usually a good sign about whether a mod is being kept up-to-date by its original creator.
