A new tree mod element type. It would have a structures page where you add/remove structure files to spawn with x/z offsets for determining when a sapling grows up.
It would have a properties page after the structures page for random tick rate multiplier to determine how fast your tree grows.
A block selector for "block/sapling" to determine which block is it's sapling.
An item selector for fertilizer item which would grow the sapling faster like bone meal, but when empty, bone meal would be default.
The 3rd final page would be triggers which would trigger procedures for example like "When saplling grows."
Custom biomes could now have multiple tree types with options for the selector being all 6 vanilla trees, custom tree mod elements and warped/crimson fungus when a generator for forge 1.16x comes out.