This is a split from D3995 to make the review easier and because I anticipate that patch to grow even more.
The idea is to make the biomes more (easily) extensible (which was suggested by @smiley D3995#187096). I am not sure if I'm using the term mixin correct, but if I've done it right, this should work in a similar way.
To showcase the new functionality I rewrote the already fixed rP26003
Compared to D1729 this seems arguably like the more complicated way to adjust biomes on specific maps, but I would think that its scales better.
Think as an example that you would want to add rain to multiple maps. Only one mixin vs mulitple biome or biome_tweak files when going with the same approach as D1729
This is of course not that straight forward as the template mixins, as you still have to change the maps to actually use the new information...
So yes, question 1 in the test is still open: