This metamagic feats allow for greater control of a spell's area of effect.
Constrain Spell [Metamagic]
You can alter a spell with a cylindrical, spherical, or conical area of effect to have a narrower area. A cylindrical area constrains to a spherical area, a spherical area constrains to a conical area, and a conical area constrains to a linear area. In the event of a spell such as Fireball that has an area of effect centered on a spot of the caster's choice, the caster also gets to choose which direction the spell travels in. A constrained spell takes up a spell slot one level higher than the spell's actual level.
Constrain Spell can be applied to the same spell more than once. A doubly constrained spell takes up a spell slot two levels higher than the spell's level, and so on.
A Constrained Fireball (Fourth Level) has a 20 foot cone area of effect. The caster selects the center point of the fireball as normal and gets to select the direction in which this cone faces.
A Doubly Constrained Fireball (Fifth level) has a 20 foot line area of effect. As above, the caster selects the point within the spell's range where this line starts, and in what direction.
A Constrained Cone of Cold (Sixth level) has a line-shaped burst area of effect. Cone of Cold always starts from the caster's hand, which is not at all impacted by this feat.
I've also written a Yield Spell (which expands an area of effect), but I wanted to see the response to this feat first. I have not played a spellcaster, so I don't know if this is insanely overpowered or completely useless. Oh well.