Is your feature request related to a problem? Please describe.
A feature I miss from overviewer was the ability to have over lays, this may require a different implementation in BlueMap such as tinting textures / only working in the 2d direct over top view but the basic idea was that you could have heat maps, biome overlays etc that would highlight the map with a colored transparency tint.

Describe the solution you'd like

• Overlays should be something you can toggle on and off.
• Overlays should only apply to the surface / maybe they are only visible in the 2d direct over top view
• Support heatmaps of player activity, overviewer supported a configurable decay from when a chunk was last updated by altering the transparency. On large maps this is very helpful to see where players have been active in the last X days.
• support highlighting bioms, when players are looking for specific resources on a large map it is nice to know for sure where those are on the already discovered map
• the ability to highlight specific block combos found under the surface, this could be used for example to draw a transit map where there is rail on gravel, or maybe just highlight all overworld cobble in the nether for walking paths.

Adding this will first require a better support for 2d-mode renders in general.
I want to head in this direction long-term, but BlueMap's main focus has always been the 3D-aspect and close-up detail :)

I'll put this on the Idea Board, but this will be on the distant-ideas column for now.