It would be a really useful to have the collision box (or sphere) of a model displayed in the editor preview window, with a toggle button, much like the display of normals for instance.
I seem to recall that this used to be a feature, but maybe I am mis-remembering.
Perhaps this is already a feature and I simply don't know how to turn on a visualization of the collision detection of a model.
In any case, this would be a very useful feature to have, and a fairly simple one to implement , as I can imagine.

Just a red box or sphere in the preview window, that displays where the collision detection is, in relation to the rendered model.