  New challenges in the modelling of galaxy spectra are bound to emerge as
upcoming telescopes like the James Webb Space Telescope will allow us to detect
galaxies at fainter flux levels and higher redshifts than ever before. Here, we
highlight three modelling problems that may become relevant for upcoming
observations in the rest-frame ultraviolet/optical at the high-redshift
frontier: stellar initial mass function sampling effects, Population III
signatures and the leakage of ionizing radiation into the intergalactic medium.
