For right now we can create trees with the simple tree structure. Sure, we can use nbt. structures to make custom tree structures, but they all are on the same height and they mostly fly above the ground. Biomes such as a vanilla Minecraft Forest also have multiple and bigger trees with a different structure. Here's my idea: Custom Trees would be a seperate mod element where you can choose either the normal tree structure we have right now, or between special tree structures which can make the custom tree have a structure like a Big Oak Tree, Swamp Tree, Taiga Tree,... (if some don't support different heights, the height option should be disabled) but you can still use nbt. files, incase if people want to create new tree structures. After you chose the tree's structure, you should then set the tree's blocks, and then finally you set in which biome the tree should spawn.
After all of that, you can create a sapling for your tree by checking a box called something like "Enable Sapling", where you choose a texture for it, and then set it's creative tab location, but the sapling isn't really necessary to add. The Biome mod element should then only ask, if vanilla trees should spawn in the biome. I don't expect that everything is going to be added at the same time, but yeah this would be really cool though! Thanks for reading!
Dangit found a related topic sry.