I would very much like to see variable stars. Periodic change of their radius and temperature (therefore, luminosity) might be a good start. (Which would require periodic change of corona radius, surface features and many other parameters, when I think about it).
Different kinds of variable stars like Cepheids or RR Lyrae are periodic, but eruptive flare stars like Proxima Centauri, or white dwarf novae are not.
There already is one type of variable stars, eclipsing binary. However, total brightness of the system does not change during an eclipse. Magnitudes of the orbiting planets change, but they drop by 30+ magnitudes...
Speaking of binary stars, I think tidal distortion of the stars' shape will be implemented (I may be mistaken, but I remebmer reading about it somewhere), like filling the Roche lobes, egg-shaped stars (or also planets and moons?), or contact binaries.
Edit: Not related to variable stars, but is it possible to implement lunar/planetary eclipse colors based on the eclipsing body's atmosphere?