You get a Fast Zombie vs. Big Daddy fights.
Edit: Also, Dog Big Daddy fights.
If only this was really possible, combining the two best FPS's into one game where Gordan Freeman must stop all the Splicers from taking over the underwater Black Mesa research facility which is located just west of Rapture. When the Splicers are to overwhelming for just one man, Jack, newly freed from his control of Fontaine goes to the aid of his silent brother. Together, they kill in silence, one using Plasmids, the other using science. Awesome yet not possible.
Made using Photoshop, Bioshock, and Garry's Mod.