"First of all, this isn't a new class. It's merely a different method of casting for a magic-user. In most of my milieu, there are dozens of different casting systems by which a worker of magic can cast their spells. Generally, characters will start out with a very basic system, much like or identical to what is detailed in the rules, and will then discover other casting systems as they travel around and experience the different cultures of the world, or worlds." - me
The system below is, obviously, a familiar one. However, it is presented here mostly in the context of starting with another system and discovering this one in the course of the campaign. If this is the case, it may be entirely appropriate to greatly increase the difference in power between spell levels, with even the first levels being of considerable power, while lowering the number of each level which the mage can commit to memory at any one time.In my worlds, this form of wizardry goes hand in hand with the sort detailed in the works of Lovecraft. While anyone might dare to peruse the pages of the Necronomicon or the Book of Eibon, and attempt to cast the spells therein, the vast majority do so at the great peril of their mental stability. There are a handful of masters, however, who have painstakingly learned the techniques of ordering their minds in such a way that the howling chaos of raw magic may be safely contained within their conciousness... for a time.
Ordered chaos is the defining character of an ephemeromancer. Through practised forms of intense concentration, he can partition a prison against madness within his thoughts. He then populates this bastion with the same spells which would erode other mens' sanity. Then, as the memories are utilised, the inverted fortress shrinks within his mind, allowing his normal psyche more breathing room.
The wizard, after allowing his serotonin levels to normalize through sufficient sleep, can then once again enforce this neuroplasticity on his mind, rebuilding his asylum of chaos.
As befitting those so learned in ordering chaos and rigidly controlling their psyche, ephemeromancers have developed a complex system of power level assessment through which they catorgorise the known spells. As one advances in power, he will know precisely how many spells of a certain power he has the skill to confine within his mind.
However, woe be to any who dare look within the ephemeromancer's books, for if they chance to understand the ancient languages in which the arcane instructions are scribed, it is sure to have a lasting effect on their sanity. Although, they may also attempt the thaumaturgy themselves, but to potentially inexpert ends.
This system of magic can use any of the multitude of published sources which divide spells into levels of power, and which then specify how many of each level may be remembered at any one time by the mage, then to be wiped from the mind upon use.