Some good stuff has been said in here about magic, but just because it's good doesn't mean it makes sense with the direction they're taking the game. In their Kickstater video they said (at about 3:50) that it's not like everybody in the entire universe is using magic, but that it's not rare. That it derives from people's souls.
Given those facts this is what I would expect of the magic in Eternity:
- It is a potential found innately in all humans. And maybe all sapient beings, creatures, or forms of life.
- Just because people have the potential, doesn't mean they ever learn how to use it.
Taking the above as given, this is what I would like it to look like:
- Magic is common, but mostly present in people, not in objects.
- Magic items, are rare and derive their power either by retaining a portion of a person's soul, or by drawing their power from the soul of their user.
- The ability to use magic does not come easily. Learning to cast a spell might be equivalent to learning to read.
- Therefore, magic is mostly used by trained specialists who rely on a handful of spells specifically crafted to make themselves way more effective. For example, a mercenary sword-fighter may devote large amounts of time to learning and improving a spell that allows him to divert missiles that are coming his way, so that he can overpower his foes in close combat. This may be the only spell that he knows, and has learned by rote, like someone who has memorized how to play one tune on a piano but can't read sheet music or improvise.
- A rarer kind of magic user is the wizard who devotes himself solely to the study of magic. A wizard understands the general principles of magic and is therefore able to cast many different kinds of spells and even improvise a little (eg. by changing an element summoning spell to summon a different element). He is able to perform metamagic, like canceling, stengthening/weakening, or redirecting someone else's spell.
- A third kind of magic user might be someone who is wealthy enough that they don't have to work and can pursue spells that strike their fancy. Already we see how this system of magic might mark the societies this world. Magic is pervasive, but belongs to the specialists and the wealthy.
- Spells might be mediated through guilds (that train in the specific and secret spells highly optimized for their profession) and universities (that teach theoretical magic rather than skill with specific spells). Thus we might have the groundwork for the traditional class system that Eternity seems to be using.
- What those classes would look like would depend heavily on what is easy or hard to do with magic. A "Warrior" in the traditional sense of a broad category of fighter who doesn't use magic at all probably doesn't make sense in this world. Anyone who fights skillfully is a warrior.