Plasma-based accelerators represent a promising advanced accelerator concept with many perspectives for their applications, for example by leveraging the interaction of plasma-accelerated electrons with electromagnetic fields for the generation of light sources with unique properties. In this Colloquium, we will first present recent progress on the development of light sources based on laser-plasma accelerators. Then, we will discuss how plasmas can be leveraged to boost the fields, which, together with high particle energies, can provide promising paths to the strong-field QED regime. Two original concepts will be presented, relying either on the fields of a laser pulse or of an electron beam, and greatly enhancing these fields by plasma self-focusing. The first concept relies on ICS based on a plasma mirror, while boosting considerably the laser intensity in a high-density plasma between the LPA and the plasma mirror via self-focusing and self-compression. The second concept uses the electron beam as the source of the fields, and a self-focusing dynamics in a sequence of conducting foils (multifoil target) to boost these fields, the beam experiencing the reflected self fields [8] at each foil that are becoming stronger as the beam is being focused, leading to the emission of high-energy photons and possibly to strong-field QED.