The ESRF storage ring, the most brilliant 3rd generation light source in Europe has been reliably supplying hard X-rays for numerous spectroscopy experiments in a variety of scientific areas. After presenting the scientific case for a storage ring lattice upgrade, the notion of the effective emittance will be introduced. The scaling of the minimum effective emittance on several optics parameters, in arbitrary lattices, will be described, based on analytical and numerical evaluation. In particular, we will focus on recent solutions for building a sub-nanometer emittance lattice in the ESRF storage ring with longitudinally varying dipole fields. Finally, design challenges will be summarized in view of present studies and future developments.