I discuss some aspects of heavy-ion collisions and, in general, of nuclear physics, about which the study of lepton-nucleus collisions as proposed in the EIC and the LHeC/FCC-eh, can provide crucial information. First, I analyse aspects related with the nuclear wave function: nuclear parton densities, small-$x$ dynamics and transverse structure. Second, I focus on items related with the initial stages of heavy-ion collisions, mainly on azimuthal correlations and the ridge. Finally, I mention the analysis of the medium created in high-energy heavy-ion collisions through hard probes, specifically on the nuclear modifications of QCD radiation and hadronisation.