Agreed, I think we all have asked for something like this in our own way on various requests.
I think the possibly properties would be something like..
Is Attractor ( attracts the elements that have Can be attracted ) attraction slider 1-10
Can be attracted: ( attracted to an Attractor element)
Is Repulser. ( repulses other elements that have Can be repulsed ) repulsed slider 1-10
Can be repulsed: ( repulsed from an Repulser element)
If an Element has both *Can be repulsed & Can be attracted, then a new slider will set which is the stronger.
If an element is not set to either Can be repulsed & Can be attracted then their other physics are applied.
All the elements can have any of the other physic properties. But an Attractor & Repulser probably should not be able to have *Can be repulsed & Can be attracted.