Tedronai: In my view, the elements already overlap. They can all attack, they can all come up with one or more blocks, and they can all do (most of) the same maneuvers.
If I wanted to add Off Balance or somesuch to a group of enemies, it could be with whipping winds or water/ice on the floor or shaking stones or spirits grasping at them from the Nevernever or anything else I could think of. It depends on the practitioner.
Harry freezes things with fire (pre Changes) because fire is his jam. Maybe you freeze things with water or gale force winds.
You could obscure an area with smoke, some sort of veil/illusion, mist, fog/condensed air or a magical sandstorm. Each of the elements has it's things it's associated with in Harry/Jim's experience, but another player/practitioner could approach them differently.
I recognize the concern that having more elements would dilute the effects associated with them, but you can already get most effects with two or more elements. Having 25 (and being able to get the same effects from five or six elements) doesn't seem like a game-breaking deal to me.
Again, this is not saying you should make your players do it. It was mostly brought up discussing how to make an elemental specialist who could advance like a Wizard. I can see players/characters who would enjoy this and players/characters who would have more fun with the 5-element system.