Variant Fly Spells
One of the ideas I plan to touch on in How to Build Spells is the idea that the core spells are the "optimized" versions of a particular spell idea, and it's possible to make variants of those spells in different schools. These variants should be less powerful compared to the "optimized" version in the core books, otherwise there's no advantage to having the "optimized" version at all and the differentiation between the schools loses its meaning.
   As an example of this idea, here are three variants of the fly spell from different schools. Each of them has a weakness, drawback, or some other element that makes them less effective or powerful than the transmutation form of fly (the "optimal" version of the spell), but they're still viable options for thematic spellcasters or specialist wizards who chose transmutation as a prohibited school.

Fly, Conjurative
Conjuration (Summoning) [see text]
Level: Sor/Wiz 3
Components: V, S, F
This spell functions like fly, except that rather than being sustained by magic, you conjure a Small flying outsider that carries the target and follows his mental commands.
   The outsider has hit dice (d8s) equal to half your caster level and 4 hit points per hit die. The outsider is a separate creatures for the purpose of all effects (such as targeted and area effects). It uses the target creature's saving throw bonuses and has improved evasion. It is subject to all effects that target outsiders (such as magic circle spells, which prevent it from entering or carrying the target into that area). If the outsider is killed, the spell ends (carrying the target safely to the ground as explained in the fly spell).
   When you use a summoning spell to summon an air, chaotic, earth, evil, fire, good, lawful, or water creature, it is a spell of that type. When you summon the creature, you decide if it is a celestial (angel, avoral, or guardinal), fiend (daemon, demon, or devil), or elemental (air or fire); this choice has no effect on the creature's abilities (though if you have an ability that augments or extends spells you cast that summon certain creatures it has the normal effect).
   Focus: A tiny bag and a small (not necessarily lit) candle.

Fly, Illusory
Illusion (Shadow)
Level: Sor/Wiz 3
Components: V, S
This spell functions like fly, except that rather than being sustained by magic, the target grows a pair of wings made of shadow that allows him to fly.
   The wings work best in shadowy illumination or darkness, where they allow the target to fly as described in the fly spell. In normal light the creature is reduced to half speed; in bright light (such as full sunlight or in the radius of a daylight spell) the creature cannot fly but still falls safely as if under the effects of a feather fall spell.

Fly, Necromantic
Necromancy [Evil]
Level: Sor/Wiz 3
Components: V, S, M
This spell functions like fly, except that rather than being sustained by magic, the target grows a pair of batlike or birdlike undead wings that allows them to fly.
   The wings have hit dice (d12s) equal to half your caster level and 6 hit points per hit die. The wings are a separate creature for the purpose of all effects (such as targeted and area effects). They use the target's saving throw bonuses, and the target's defenses (such as protection from energy) apply to the wings. The wings are destroyed, the spell ends (carrying the target safely to the ground as explained in the fly spell).
   The wings are vulnerable to turn and rebuke attempts. If turned, the wings flee (bringing the target with them) on the target's turn for the duration of the turning. If rebuked, the wings cower and refuse to move the target from its present location (though this doesn't prevent the target from using other methods to move or performing actions). If commanded, the commanding creature can direct where the target flies but does not otherwise control the target.
   Material Component: A black onyx gem worth at least 1 gp.