One thing I was thinking of exploring was “spirits”. A lot of spells in Superno state that the mage must “trace a circle, and write the names of three spirits” or “he must write the name of a special spirit on the object” etc… I think it is obviously good to assume that a mage would have knowledge of perhaps “generic spirit names” usable in their spells, and not complicate things further. Having said this, it could be fun to actually create a library of spirit names and sigils which a mage could learn and add to spells. Adding one of these more “advanced” spirits names to a spell would require a roll, perhaps mnemonics, or calligraphy etc…, with success allowing a bonus on the actual spell roll, or some minor benefit. It could also be possible to create a system by which a mage could call on a spirit for protection at any given moment, perhaps under the form of a memorized first level spell (“Invoke spirit”?), and correctly calling the spirit and tracing his sigil would provide some minor protection.
Learning the name of one such spirit would require some time, perhaps as a function of the power of the spirit in question, vs. the level of the Mage, and of course, it would be contingent on having either a teacher capable of teaching it or a book containing the information. Learning the name/sigil would involve memorizing the name and the correct invocation words, as well as memorizing and being able to correctly mentally visualize the sigil.
There is a lot of material to plunder… erh… get inspiration from and establish a list or library of such spirits with minor info on each so the player and DM can decide on which spells or rituals they may be used. And this list could be connected with specific books. The Mage might make it a goal to find copies, etc… to learn specific names. Many spirits are connected to the planets, for example, and could be used, with their sigils correctly traced, as a substitute for a talisman.