Animatable particle density

It would be nice if particle density could change with an animated vertex group or a texture. Currently, particles spawn from a static position even if you have a vertex group that changes frame by frame, like from a Vertex Group proximity modifier or with dynamic paint.

A few examples would be like a brush making bubbles on a surface or steam rising from where a hot object touches a damp surface. It could have many uses.

Please and thank you for reading my idea.