The Montane forest and plains

The montane forest is found in mountains, but not as much on steeper mountains; rather, on large, mesa-like plateaus. In these, both spruce trees and a new type of tree, the beech tree, are found. Here are some examples of beech textures for colours: 
https://imgur.com/gallery/6vjxDW5
These new colours could be useful for building.

A new type of stone is also found in this biome and the mountains surrounding it: slate, a dark blue-gray rock, craftable into polished slate, slate bricks (the same sort as stone bricks), slate tiles (like a slate version of normal bricks, useful for roofing), and all the derivative blocks. This would be useful for building. 

Montane plains would just have less trees, basically.

Here are some mob ideas for the biome:
Goats are found more often in montane plains and on mountains rather than the forests. They drop goat's meat which gives three full hunger things. When sheared they give 1-2 wool, with a very small chance to drop their horns. If this happens, the goat takes some damage as well. The horns can be used for a horn banner pattern and bonemeal, and maybe helmet decoration. Goats can be used for goat's milk, which makes cheese. Cauldrons could also be a part of this.
Deer could also be found here.
Hawks (or similar) are found here, with a hawking mechanic which brings rabbit or chicken to the player if befriended or tamed. They'd have nests & eggs.

Perhaps there could be a new mountain ore.