GDevelops current behavior creation system have some flaws, which is mostly not being able to pass other objects into the events of the behavior, so if you are making a platformer/physics controller type behavior there is no way to check for collisions with platform/physics object. I propose a new type of behavior system where you can make classes, sort of like object constructors in javascript. You would code for the class, just the general code, and for different objects that would have the class, you can attach parameters (like if you are making a 2 player platformer game from scratch you can attach the controls as parameters). This behavior would only be able to be used in the project it was created. Feel free to ask questions in the comment.