Yet again a bit late, but next title is standing for elementals. Usually in roleplaying games elements are basics in magic and types of elemental entities. There are four basic elements; water, fire, earth and air used in both magic and elementals.
But there might be additional elements and elementals in addition to those basic four. These might be for example light and darkness and more exotic ones.
Elements tell the nature of magic, for example. Fire could be damaging, earth protecting, water healing and air for statuses. There might be different schools in magic (good example of this is Dungeons & Dragons) when elements aren't in that great focus.
Still, how elements could work? If designing a game system where elements are present, you could consider relationships between the elements. For example same element against same element won't necessarily be that powerful, but opposite elements can be powerful over others. For example water element could be powerful against fire element, and fire element against earth.
I think, that if magic system is based on elements, you could design it the way that each element covers different kinds of magic, but some elements are more powerful or have more access to certain type of magic.
For example, even though water is healing element, it can also bear powers of damaging and protection. But it's main focus is in healing. Similarly even though earth is focused and good on protection, it can also be used to damage but it is not that effective.
This focus of elements can be easily carry out in spell lists. In fire spell list majority of spells are of damage, few of protection and only minimally for healing. Water could be focused in healing, can also damage but minority of spells are about protection.
I could also determine, that all in one element mage is more powerful than mage using two or more elements. Reason is, that mage focused on one element knows it best, when mage focusing on multiple elements doesn't have similar intensy as he has to divide his time and learning to two or more elements of magic. One element mage might be more powerful, but mage who knows more elements is more versatile though weaker.
To determine power over different elements, you could give a value for each element in character sheet. When mage gains a new level, he can share points to his elements. It is clear, that if he uses all his new points to one element, he will be more powerful than if he would share the points between elements. Points in elements could mean mage's level in that element or magical power in that element. More points he has in one element, better he is casting them or he has wider access to spells in that element.
Element Magic In D&D
Basically, if you want you can turn D&D into element magic system. Just take spell list, share spells in elements and make four schools of magic, earth, fire, water and air. Each element mage is it's own class. This way you can multi-class to other elements and it works fine with other rules. You know more spells in wider range, but aren't as powerful. Even though your character is level 10, but is level 3 fire mage, level 3 water mage, level 2 earth mage and level 2 air mage he is more versatile than level 10 fire mage, but level 10 fire mage is way more powerful in his spell range.
Summon spells could be accessed to every element mage, but what creatures they can summon could be element tied.
In my opinion there are no element sorcerers, as sorcery comes within, not learned.