I think we need more 2×2 Giant Tree variants in Minecraft. I built few to see what it could look like in Minecraft.

I apologize if these don't match your expectations. According to this idea Yellow Birch would be a Giant Birch with yellow leaves that can be found in Yellow Birch Forest biome where Tall Birch trees have yellow leaves and these giant trees can be found among them. There would be new flowers in this biome. I think I made Giant Acacia a little taller than it should be.