>Select two spells.
>These two spells are combined to create a new spell
>You may cast this new spell once per day
What'd be the most potent mixture?
Nah m71, I use Wish to Wish I could combine Wish and Wish into Doublewish, and as per OP I can cast the newly created spell at will 1/day, which means I immediately am allowed to cast my Doublewish, and forever after once per day.
>What'd be the most potent mixture?
Magic Missile and Prismatic Sphere...
Ummm - Prismatic Missiles. Weee!
My other thought was 'Chain Lightning' and Fireball for...
Prestidigitations are minor tricks that novice spellcasters use for practice. Once cast, a prestidigitation spell enables you to perform simple magical effects for 1 hour. The effects are minor and have severe limitations. A prestidigitation can slowly lift 1 pound of material. It can color, clean, or soil items in a 1-foot cube each round. It can chill, warm, or flavor 1 pound of nonliving material. It can create 2- foot-diameter spheres spring from your outstretched hand and streak in straight lines to the spots you select. The meteor spheres leave a fiery trail of sparks.
If you aim a sphere at a specific creature, you may make a ranged touch attack to strike the target with the meteor. Any creature struck by one of these spheres takes 2d6 points of bludgeoning damage (no save) and receives no saving throw against the sphere’s fire damage (see below). If a targeted sphere misses its target, it simply explodes at the nearest corner of the target’s space. You may aim more than one meteor at the same target. Once a sphere reaches its destination, it explodes in a 40-foot-radius spread, dealing 6d6 points of fire damage to each creature in the area. If a creature is within the area of more than one sphere, it must save separately against each. Prestidigitation can create small objects, but they look crude and artificial. The materials created by a prestidigitation spell are extremely fragile, and they cannot be used as tools, weapons, or spell components. Finally, a prestidigitation lacks the power to duplicate any other spell effects. Any actual change to an object (beyond just moving, cleaning, or annihilating it) persists only 1 hour.
Sphere of annihilation
Leomunds tiny hut
I'm running a gestalt campaign with a fusion mechanic, and I am totally stealing this for the fusion prestige class. Currently the way the class goes is, 1st/3rd/5th level - you contribute a move/attack action to the combined being, and then on the 2nd and 4th level you get to choose from a variety of tricks like faster/longer fusion time. This is going to be a trick for sure.