Even though saplings can be added manually, it may be good to add native support for a few particular mods:

1. Tinker's Construct (Slime Saplings): Popular mod, unique "trees" that would be an aesthetic twist as a bonsai.
2. Natura: Semi-popular with some wonderful trees. Special support would need to be added for the bloodwood sapling, which goes on a ceiling. Upside-down bonsais please!
3. Plants: An up-and-coming mod that adds a few of its own trees. It doesn't require any special support, but would be nice to have integrated automatically.
4. Rainbow Oaks: Adds one tree that has a rainbow gradient pattern. Some special support might be required.

• Tinkers Construct Saplings ✔️
• Natura Overworld Trees ✔️
• Natura Nether Trees: We'd first need a feature to let certain sapling require certain conditions, i.e. they can only be used when the Bonsai Pot itself is in the Nether.
• Plants: I'll take a look soon.
• Rainbow Oaks: It would probably look always the same since the leave block positions are always the same. Maybe try this with a simple JSON integration and if it works create a Pull Request?