  We discuss the parameter space reach of future experiments searching for
heavy neutral leptons (HNLs) at the GeV scale in terms of neutrino mass models
with three HNL generations. We focus on two classes of models: Generic
assumptions (such as random mass matrices or the Casas-Ibarra parameterization)
and flavor symmetry-generated models. We demonstrate that the generic
approaches lead to comparable parameter space predictions, which tend to be at
least partially within the reach of future experiments. On the other hand,
specific flavor symmetry models yield more refined predictions, some of these
can be more clearly excluded. We also highlight the importance to measure the
flavor-dependent couplings of the HNLs as a model discriminator, and we clarify
the impact of assumptions frequently used in the literature to show the
parameter space reach for the active-sterile mixings.
