I really like to see something like you build a gauge and spend all on your "new form", like a Devil Trigger of some sort.

Like you use skills and "avatar OGCDS" to build a special bar, when it's 100% you can activate the form where you transform with your void like in the trailer. The trick is you have to know when you should activate it or not, something like this.

Nothing so complicated, but very 'flamboiant" as they mentioned it will be.