This mod replaces each birthsign with a new one with new powers and abilities. I felt the vanilla birthsigns didn't add that much to the gameplay, so this mod attempts to make each birthsign unique and creative, capable of making each playthrough feel different without making the player too overpowered.
This mod does not alter the birthsign images. As you only see them in the beginning of the game, I didn't think it was that important.

The birthsigns are as follows:

The Warlock
"Chaotic and unpredictable pranksters, those twisted by the Warlock can become invisible at will and once a day can turn entire armies insane for 60 seconds, causing them to attack everything and everyone. They can also summon a highly dangerous sheep that will attack anything it sees, including themselves. Impossible to contain, Warlocks are very lucky and immune to paralysis."

The Elemental
"Those born under The Elemental are immune to fire, frost and shock but have a 100% weakness to normal weapons."

The Healer
"The Healer's Blessing grants a 10 point bonus to Willpower and Restoration. Once a day those born under the Healer may completely restore all attributes. They also gain powerful healing spells, capable of healing their allies as well."

The Prophet
"Those born under The Prophet have been chosen by The Nines. Once a day they can invoke Divine Protection, becoming invincible for 60 seconds. They can invoke the Holy Flame to banish undead. They can create light, walk on water and through faith fortify their health and shield from damage."

The Loved
"Everyone seems to like those born under the stars of The Loved. They gain large bonuses to Personality and Speechcraft. They can make others vulnerable to poison and summon a deadly dagger at will."

The Sorcerer
"Those born under The Sorcerer gain an 80 point bonus to magicka and restore magicka much faster than normal."

The Dying Star
"Those born under the Dying Star excel in dark magic. They can reanimate dead corpses at will, trap souls and summon a plague rat to assist them. Once a day they may summon a powerful raging spirit for 30 seconds." (The summons provided do not follow you and will attack everyone who isn't you.)

The Demon
"Charming and malicious, those born under the Demon can command a human to do their bidding once a day for 60 seconds. They can charm people at will and strike fear into the hearts of weaker enemies. Those tainted by the Demon are immune to fire but vulnerable to frost."

The Darkness
"When no stars shine down on Tamriel, those who are born then are instead touched by Sithis. Once a day they can terrorize their enemies, or absorb their life energy. They gain powerful spells of death, immunity to poison and large bonuses to sneak and speed, but those who are touched by the Darkness will die instantly if exposed to sunlight."

The Beast
"Those born under The Beast have an affinity for nature. They gain bonuses to Endurance, Alchemy and Alteration. Once a day they can temporarily control any and all creatures in a 20 feet radius or go into Beast Mode, fortifying Endurance and Strength. They can summon a bear and command low level creatures at will."

The Assassin
"The Assassin grants bonuses to Agility and Sneak. Once a day, those blessed by The Assassin may become completely hidden for 30 seconds, able to rob and murder unseen. If discovered, they may also paralyze all opponents around them for 20 seconds for a quick escape. Their Long Reach lesser power can snatch objects from up to 60 feet away."

The Spire
"Standing tall and proud, those blessed by The Spire have a 25% shield surrounding them at all times and restore fatigue faster than normal."

The Smasher
"Those born under The Smasher have a keen ability for smashing, gaining a 10 point bonus to Strength and Blunt. Once a day they may summon an unstoppable warhammer to destroy their enemies."

Compatibility
Incompatible with any birthsign mods that alter the vanilla birthsigns. It should be compatible with mods that add new birthsigns, such as Blademan's New Birthsigns. Some of the birthsigns here were inspired by his mod, which I recommend checking out!
When I tried loading this mod with an existing save, it crashed my game. Works fine with a new game.
This mod has been cleaned with TES4EditQuickAutoClean.

Install
Just extract the UniqueBirthsigns.esp file into your Oblivion/Data folder and activate it in the Oblivion launcher.