Detailed simulations of the gamma-ray sky played an important role before the launch of Fermi. Pre-launch simulation campaigns aided the development of new analysis tools and the assessment of the expected Fermi-LAT science performance. After four years of the Fermi mission in orbit, sky simulations can still be very useful to support the activities of Fermi, to refine our understanding of the scientific performance and to explore potential new science cases. We have used the LAT observations collected so far to develop new, highly-detailed simulations of the sky as seen by LAT. These simulations include the isotropic and Galactic diffuse emission, as well as the list of point and extended sources detected by the LAT. Furthermore, our sky simulations account for transient and variable sources, including Gamma Ray Bursts, pulsars and flaring blazars. We report on the latest developments of this new sky simulation work, describing its potential benefits for improving the science of Fermi today and in the near future.