I don't have a deep desire to see event behaviours, mainly because it feels doable right now, and I envision the downsides of what we have right now would also affect event behaviours.
Say you made a custom platformer movement in events for a "Player" object, all in 1 group,and then you want to copy this to a new project? Well then you better have any objects/families with same name and same variables in other project before you can copy/paste this group. Once everything exists then you can indeed copy and paste.
Let's assume we have behaviour event sheets... Same issue, any "on sprite collide with ground", the new project will need that ground sprite, any vars from the ground sprite need to be added, etc, before you can copy that behaviour to a new project.
Maybe the more reasonable solution would be a better copy/paste system, like a special paste or import/export functionality that, instead of attempting to paste and having C3 say "cannot find sprite, cannot find var", it would automatically know to copy the relevant objects/families across. Maybe being able to export a group of events as JSON which captures all relevant objects along with instance vars, effects, and their defaults, so that they can be imported into a new project.
I sorta see the idea of "applying an event behaviour to any object" but then this is also achievable with families, throw a new object into the family and boom, the group of events applies to them.