  We review open questions and prospects for progress in ultrahigh-energy
cosmic ray (UHECR) research, based on a series of discussions that took place
during the `The High-Energy Universe: Gamma-Ray, Neutrino, and Cosmic-ray
Astronomy' MIAPP workshop in 2018. Specifically, we overview open questions on
the origin of the bulk of UHECRs, the UHECR mass composition, the origin of the
end of the cosmic-ray spectrum, the transition from Galactic to extragalactic
cosmic rays, the effect of magnetic fields on the trajectories of UHECRs,
anisotropy expectations for specific astrophysical scenarios, hadronic
interactions, and prospects for discovering neutral particles as well as new
physics at ultrahigh energies. We also briefly present upcoming and proposed
UHECR experiments and discuss their projected science reach.
