Actually I think it’s nice that different maps have different biomes. It helps to make them feel unique. If every map could have it’s biome chosen ahead of time, I think they’d tend to blend together more.
I can understand people wanting more biomes for Arabia (and maybe Arena, BF, and Nomad). But I like that the other maps are specialized.
This is a nice idea, and I’m pretty sure it would be possible to implement for map scripts that were written specifically to support it.
But there’s no way it could work for all map scripts, especially not custom ones, because the script would have to be written in a specific way. AoE2 doesn’t restrict you to specific biomes/tilesets when making maps/map scripts (which is a good thing).
Arabia, yes. Coastal was (still is?) the official default map, but it’s generally known for being unbalanced. I think it’s a shame, and that Coastal would be a great standard map if it was balanced, but unfortunately the distribution of terrain and resources is too random for it.