DMDynamic's page

Combining the effects of spells is something that I think has been overlooked in the base game. Even with the Arcanist hybrid class, it doesn't actually combine spells, just uses spell slots to cast spells quicker. An idea I had that I am using in a campaign I am running is a series of 3 feats I dubbed Spellsmith, Advanced Spellsmith, and Master Spellsmith.

Spellsmith prerequisites:
Must be able to cast at least one 2nd level spell or higher
Must have at least two spells prepared/available belonging to different schools one level lower than the highest known spell level, excluding cantrips

Spellsmith works by combining two spells from two different schools of magic in a way indicated by the caster at the time of casting. The casting time for the combined spells is equal to the spell with the longest casting time, while the duration is equal to the shortest spell's duration. The caster must expend one spell slot equal to the highest level of spell being combined and one spell slot of one level higher, for balancing of course. Then, the caster makes a spellcraft check with the DC equal to either 10 or 15 plus double the combined spells' total level. If the spellcraft check is less than half the DC, the spell fizzles and fails to cast, effectively wasting the spell slots. If it is between half the DC and the DC, the spell combines in a way determined by the GM. And if it is equal to/beats the DC, the spell combines in the way intended by the caster. The only major restriction with spellsmith is that it applies to spells up to 4th level. This can be used up to 1/4th the caster's hit dice times per day.

Advanced Spellsmith prerequisites:
Spellsmith feat
Must be able to cast at least one 6th level spell

Advanced spellsmith works the same exact way as spellsmith, except the max level of spell combined is increased to 8th level, and the caster gains an additional use of this ability per day.

Master Spellsmith prerequisites:
Spellsmith and Advanced Spellsmith feats
Must be able to cast at least one 9th level spell

Master spellsmith improves the spellsmith feat by costing only one expended spell slot of one level higher than the highest combined spell per use. Additionally, cantrips/orisons can now be combined with other spells using spellsmith, and combining two cantrips/orisons does not expend a use of the spellsmith ability.

Lore-wise this feat is a result of years of magical research at a college for wizardry in my campaign's world, and only characters who studied at that college could take the feat and it would only apply to arcane casters, but for the sake of fun I allow other casters to take these feats as well regardless of whether they studied there or not. If anyone wants to make use of these feats and test out how they work in practice, feel free to reply with your results, be they success or failure.