What's still kind of limiting about it? How is adding it as a behavior more limiting than ticking a checkbox?
Its not, this actually could fix a lot of issues.
Like the fact that center view on me is pretty much useless as an attribute if you have more than one object you want to follow. Same goes for solid, adding or removing the attribute was always buggy in C0.xx.
More importantly I think this will add the possibility for many more types of attributes, like an outer bounding box attribute for example.
Unlike solid or, per pixel, or bounding box, outer bounding box would only detect collisions around the perimeter of the object, IE no collisions within the box.
Instant borders from one object instead of 4.