  The high-resolution X-ray spectrum of the Perseus galaxy cluster observed
with the Hitomi satellite challenges astrophysical collisional ionized plasma
models that are widely used in the community. Although Hitomi spun out of
control, several Hitomi-level missions have been proposed and some funded. The
spectrometers aboard these future missions have a broader energy range and/or a
higher spectral resolution to achieve different scientific goals. Accurate
plasma models and atomic data are crucial for plasma diagnostics of
high-quality spectra. Here, we present a few cases where improvement of plasma
diagnostics will be decisive to better understand celestial bodies and their
physical processes at play. We focus on collisional ionized and photoionized
astrophysical plasmas in the context of developments of plasma models, as well
as current and future generations of spectrometers.
