This may sound similar to other ideas but it’d not. The desolate forest is a biome that is almost devoid of all mobs. It has black trees that are different to normal vanilla trees, because instead of leaves they just have sticking out branches. The tree would be called charred tree, and could be crafted into charred logs, planks wood stairs etc. it would make a great building material for modern or spooky houses. It would also have a large blue fog covering the entire biome and travelling 59 blocks into the air. This could not only be something interesting to play around with (using command blocks) but it could also add an almost eerie vibe to this biome. It would also have souls floating around, as it was once full of life.
Structures wise, I think it could have a graveyard. For some of the graves, it could have loot, but not many as that would be overpowered. I think this could add for a fun challenge of trying to locate the chests. I think it should also have a different form of spawner, like a set of blocks in the floor that make zombies and zombie villagers spawn more frequently. I also think that if you die, it spawns in an armoured zombie (possibly even a bob) to add an element of challenge if you want to fully conquer it.
Finally, I think the water should give you food poisoning, as it would be stagnant. I think this would be a great biome for minecraft.