The CERN SuperProtoSynchrotron (SPS) represents an ideal facility for fixed-target heavy-ion experiments exploring the phase diagram of strongly interacting matter in the region $200\lesssim\mu_{\rm B}\lesssim500$ MeV. It can deliver high-intensity beams ($>10^6$ Pb/s), allowing a study of rare probes of the Quark-Gluon Plasma, including electromagnetic and hard processes. The NA61/SHINE experiment is currently active and plans to perform a first direct measurement of open charm production in Pb-Pb collisions at top SPS energy and possibly at lower energies. The project of a new experiment, NA60+, based on a muon spectrometer coupled to a vertex spectrometer is currently being developed, for the study of dimuon and heavy quark production, and a Letter of Intent was recently submitted. In this contribution the physics motivation for the studies of rare probes, the existing and planned experimental set-ups and their expected physics performance will be discussed.