So, I was sitting around thinking. I came up with this idea. I don't know exactly what to call it but here I go:
First off a bigger difference in heights. Maybe valleys and bigger hills.
Ok, so we know there are hills/mountains in Minecraft.
What if you could have a big forest biome on top of one? Kind of like a biome in a biome. Most mountains in MC aren't very wide at the peak maybe 10x10 at the most idk. What i am suggesting is more like a very wide mountain with a forest on top. That is my second part of the suggestion.
EDIT: also there could be river biomes on mountains/ valleys etc.