Ok to keep confusion to a minimum I am just going to have the base 8 elements and the non-elemental element as well.
Fire: This is fire people, it hot, it burns, and it pretty. This does include magma as magma will burn things just as well if not better than a flame, so enjoy making fireballs and large globs of magma.
Ice: It cold and it freezes things into easy to shatter blocks. With this you can do what ever you want with ice, snow, or sleet. Have fun with your winter wonderlands.
Wind: This is wind my children, it blows really hard and it can cause storms that can lift a house off its foundations. This ALSO includes making sound base attacks as sound need air to work and wind is manipulation of air.
Earth: This is the element of earth, it hard it heavy and it not fun to get dropped on your head. Earth can be used to make earthquakes, throw boulders at people, make metal objects be flung at someone, and yes you can use this to control plants to make them do your bidding.
Thunder: This is the element of thunder, it fast, it shocks you, and it hurts badly. This is all about using the element to bring pain to your foes, do what you want with it.
Water: This is the element of water, you drink it, you use it for wet T-shirt contest, and you use it to bring ruin to your foes. This also includes the manipulation of blood in a person's body as blood is made mostly of water.
Light: This is the element of light my friends, this element is used to smite the wicked, heal the wounded, and make it easier to read your stories. With this you can heal, you can deal damage, and you can blind someone with bright flashes of it.
Darkness: This is the element of darkness this is one of the more feared element due to its nasty nature. The element can be used to harm people, inflict the zombie status on people, and also helps one poison others.
Non-Elemental: This element is the ONLY element to have no strengths or weaknesses, it does the same amount of damage to EVERYONE. This is where powers that don't fit in the other elements go, one example is draconian powers as Bahamut is an avid user of non-elemental attacks.
V Elemental strengths and Weaknesses V
Also No one and I mean NO ONE can have Light and Darkness for their elements. It that way only because you wouldn't have any weaknesses if you have them both.