Per one PC's request, I have looked into adopting the spell modification rules found in the Nightbane sourcebook Through the Glass Darkly. I have decided I will allow them, with the following modifications:
1) No brand-new spells can be created. At this time, the only implementation I am allowing is for the modification of the effects of existing spells. As per the book, a modified spell will be 1-6 levels higher than the original spell.
2) The chance of success will be determined normally. However, since virtually no classes outside of Nightbane have a Principles of Magic skill, I will be replacing it with the Lore: Magic skill. An additional bonus of 10% for all "traditional" spell casters, 5% for techno-wizards will apply. Warriors who have been trained to use spells, supernatural creatures who happen to have spellcasting abilities, and the like receive no bonus. So, the modified formula for success is as follows:
IQ+ME+Caster's level+1/10th of Lore Magic skill+bonus as outlined above = percent chance of successfully modifying the spell.
A +10% bonus applies if the modifications to the spell make it less potent, or if said modifications a VERY MUCH in line with the spell's basic function. An additional bonus may be added at GM discretion if the PC is operating somewhere conducive to spell research (a well-furnished laboratory, or a ley line, for example).
3) There is a -10% modifier for each level of the spell being modified above level 9. There is a flat -50% modifier if the spell is being made more potent, or altered in a way that is significantly counter to the spell's basic function. This is a slight departure from the rules as written. Specialist magic classes suffer a -20% penalty when modifying spells not within their particular specialty (for example, a temporal wizard cannot modify a conventional spell without this penalty).
4) The time to successfully modify the spell remains the same, two weeks per spell level.
5) If the roll fails, the GM will apply the random effects table liberally.
Note: EP-purchased automatic successes cannot be used on spell modification rolls.