Some of vanilla trees have variants that can naturally generate in world, but can't be grown from saplings:

1. Tall Birch from Birch Forest M and Birch Forest Hills M biomes
   

2. Oak variant from Swampland biome
   

3. Pine-like Spruce variants from Taiga biome
   

It would be great if those tree variants can be grown from vanilla or special craftable saplings (like it was done in Garden Stuff mod: 1 2 3)

This is a decent idea. I'd have to look into how vanilla handles those trees. I am not certain I'd want to make them their own sapling as that feels a bit less immersive, probably make them a rare chance like the large oak trees.