This is why more templates would be good. A behavior node I can’t edit, or have to work around does me no good if I want it to behave different. What if I want to change the jump behavior Node’s height, it’s double jumps, wall jumps, jump height after a wall jump but before double jumping?
It sounds good! I want control, and I know how frustrating it can be not having it, even for beginners. Most people on the help forum come with issues from merging tutorials, maybe that’s a positive for decorators 
as long as they are truely extensible.
I do believe making enough high quality decorators is a daunting task, based on the examples below. And you likely wouldn’t make any money from it in the godot eco-sphere.
As for already implemented things, I think the closest you’ll find for Godot so far would be the Block Coding. It’s still code, but the actions are simplified and they have some edited Charater controllers samples.
I recall Unity having a similar idea. This “Game Creator” plugin boasts a huge toolbox of components, I think that’s what a decorator plugin has to aim for. Rounding out as many edge cases as possible.
Tangentially C++'s standard library often lacks, so the boost library is a massive collection that more than doubles the stl’s uses. Every non-game C++ developer I know uses boost.