.gpx track must have timestamps
Incorporating some of the overall sky colors might be neat, so that twilight looks different than midnight.
Relevant to the parent suggestion, you might add some indicators on the slider to show where daytime is.
Mapbox has paid DEM data to zoom level 20, but I think it's just zoom level 15 scaled up (but the data is cleaner and more recent) 
Nextzen also open-sourced their elevation tile generator, but I haven't tried to generate my own tiles 
If you're interested, there are more features like the ability to view shade for any time/date/location throughout the year, search, building shadows, and high DPI mode available at the base url: https://shademap.app