Here is my thought.
The Main Physical Elements
Have 4 Main Elements: Water, Fire, Air, and Earth
Have the new characters (or old named characters) be users of those four main element and are the original members of the Toa team (similar to how the Justice League has its own originating cast of characters). They are the Elite 4, the master of their specific craft, the best of the best for the specific elements.
Later on you are able to add new characters into the mix…those characters literally having mix abilities of the 4 main elements (aka Sub elements).
The Main Sub-physical Elements:
Those would be:
Wood (aka Jungle): Subelement of Water and Earth
Magma: Subelement of Earth and Fire
Cryo: Subelement of Air and Water
Lightning: Subelement of Fire and Air
Smoke: Subelement of Fire and Water
Sand: Subelement of Air and Earth
The Main Specialized Physical Elements:
Some could also be specialized users from the 4 main elements:
Metal: Specialized users of Earth, more precise in their Earth usage
Healing: Specialized users of Water, more benefitial and regenerative Water usage
Sonics: Specialized users of Air, more creative users of the sound transfering properties of Air
Plasma: Specialized users of Fire, more heat conducting Fire usage
All of these specialized elements, subelements, and main elements are part of the Material Elements, those that shape the worlds physical aspects. But then you have the Ethereal Elements, those that shape the worlds etherial and non physical aspects.
The Main Ethereal Elements:
The main 4 Elements here would be: Light, Time, Spirit, Darkness
They also have their subelements and specializations, but rarely would anyone incounter users of these elements, since the Elements shape the Ethereal part of the world
The Main Sub-Ethereal Elements:
Void: Subelement of Darkness and Spirit
Aether: Subelement of Light and Spirit
Psionics: Subelement of Time and Spirit
Chaos: Subelement of Time and Darkness
Order: Subelement of Time and Light
Mana: Subelement of Darkness and Light
The Main Specialized Ethereal Elements:
Magic: Specialized use of Spirit, one so intune with the Spiritual energies of the world, that is able to shape it through the interior Ethereal energies of the world. Reshaping it as he shooses.
Life: Specialized use of Light, one is in tune with the living energy of the world, as such is able to manipulate the creation of living things.
Death: Specialized use of Darkness, one is in tine with the entropic energies of the world, as such is able to manipulate the destruction of living things.
Dimension: Specialized use of Time, one is in tune with the reality of the material and ethereal worlds that is able to recognize other types of worlds and travel across them.
This will allow you to have Heroes and Villains of all types of elemental abilities under these categories. While the 4 main physical elements are the introductionary ones (most likely used by the main protagonists), the others can be used by additional protagonists and antanogists. Some elemental user are even indifferent to the concepts of morality, most notably from the Ethereal branch of Elemental usage.