So here's my idea.
There are two sources of magic in the world. The world itself, and the user of said magic. These two sources synergize to allow the user to use the different forms of magic available to them. At different skill levels, multiple things can happen. There could be areas that are completely cut off from the world's "Field" of magical energy, yielding lower-level mages nearly useless, as they rely heavily on the worlds magical field. But as a mage becomes stronger, they can become more dependent on their own magical source, and if one is EXTREMELY powerful, or there are a large group of relatively powerful ones, they could bounce their magical fields off of each other or themselves, allowing for greater magic to be used.