  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, improved 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.
