The Problem

So, there is a Feature that World Edit could Possibly Improve on and this is the Bone_meal item for Brush Command goes as follows.
/brush apply cyl item bone_meal
so, We are all aware that bone_meal is used not just for farms but also decoration since we can use it on new blocks like Mossy block. but it covers all the Area which is 100% by default and each block.

A Solution

The solutions I was thinking how blocks have the 10%stone,90%grass_block for example. that only bonemeal or other blocks that you right click with can have a % added to them.
this would give it a more natural look, since you can make it 50% or 1% depending on how you like it.|

Alternatives

Other solution is doing it Via hand or using the Blocks under and changing the style.
but this would require more steps.

Anything Else?

I'm also aware that bone meal works under the water, so this also gives it an extra chance.
I don't know if World Edit has Alternatives, but //flora is a region and you need to find Pos1 and pos2 and lets not forget World edit does not support //flora for the Mossy blocks. while bone_meal is part of the game.

another thing I was thinking was... Small schematics (like custom Trees) added in a way that you can make a bigger bone meal that generates the trees like World Painter (Just an Idea)

From what I can tell everything you've asked for already exists?

You can just use a noise mask to reduce quantity

And there's the schematic brush

I'll do some more Research into it then, what would the noise command look like for example?