This would probably be quite a challenge to complete but I've seen it done before to some degree. I would like to see the ability to assign different arrays of saplings, different tree types. This could be as simple as a 2x2 oak sapling square (I understand something similar was mentioned) or something a bit more complex like a 2x2 of birch and dark oak saplings. This way you could have all of your basic tree types as custom schematics while say adding a cherry blossom as a birch and oak combination or a willow tree as a spruce and dark oak combination. This just opens up more possibilities for the plugin and makes building custom trees so much easier. Whether or not this would be possible to even configure in a config file or even code, I have yet to fully explore myself but I assume the config could look something like:

combinations:
birch-oak:

• birch-oak/1.schematic
• birch-oak/2.schematic
• birch-oak/2.schematic

jungle-oak:

• jungle-oak/1.schematic
• jungle-oak/2.schematic
• jungle-oak/3.schematic

But that's up to you, it would take a lot of effort to create something like this I assume as it's no longer replacing the natural tree generation. If you think you need help with coding any of it, hit me up, I'm new to plugin creation but I wouldn't mind giving it a whirl. Also, I can provide the default schematics if necessary, just reach out to me.