That's a good question. I would think it's Zenescope. They have a large expanded universe in their Grimm Fairy Tales line. Belle, Cinderella, Three Musketeers, Robyn Hood, etc all slaying monsters in their own worlds but that tend to cross over for special events.
Action Lab is trying to create a shared universe, but all of the books are written by one guy and they are pretty bad, with tits and ass taking the place of any coherent storyline.
Other than that, I'm not sure. Image Comics used to share a lot of characters. Youngblood and Spawn for instance, but that hasn't happened in the regular series books for years.