As I mentioned in the introduction and magicka system, I have plenty of things finished already, including birthsigns. Here's what I came up with for those.

Birthsigns
A Birthsign in Tamriel is a constellation under which a person can be born. A person born under a sign is bestowed magical bonuses, ranging from increased magicka to a paralyzing kiss. These bonuses can be perceived as either a blessing or a curse. Birthsigns cannot be changed willingly. The only time a birthsign can be changed is if a character is reincarnated using the spell Reincarnate. The new birthsign cannot be chosen, instead the character is given the birthsign that applies to the season in which the character was reincarnated. All birthsign bonuses are applied after racial bonuses, but before all other bonuses.

The Apprentice
The Apprentice's Season is Sun's Height. Those born under the sign of the apprentice have a special affinity for magicka of all kinds, but are more vulnerable to magicka as well.
Elfborn: +2 bonus spell slot equal to half your level(minimum 1, maximum 9) or lower. Gain weakness to magicka.

The Atronach
The Atronach is one of the Mage's Charges. Its season is Sun's Dusk. Those born under this sign are natural sorcerers with deep reserves of magicka, but they cannot generate magicka of their own.
Wombburn: Double your total spell slots. Spell slots are not regained by resting. When hit by a spell or spell-like ability that would affect you, even if energy resistance you possess would reduce the damage of a spell to 0, roll 1d20. If you rolled a 15 or higher, you regain one spell slot and negate the spell's effect and damage. The spell slot regained must be of an equal level or lower than the spell you absorbed. If you have not used any spell slots yet, this effect does not activate.

The Lady
The Lady is one of the Warrior's Charges and her Season is Heartfire. Those born under the sign of the Lady are kind and tolerant.
Lady's Favor: Permanent +1 bonus to Charisma score.
Lady's Grace: Permanent +1 bonus to Constitution score.

The Lord
The Lord's Season is First Seed and he oversees all of Tamriel during the planting. Those born under the sign of the Lord are stronger and healthier than those born under other signs.
Trollkin: Gain weakness to fire.
Blood of the North: Regain 4 hit points + half your level(minimum 0) per round for 3 rounds + ¼ your level(minimum 0). This spell-like ability can only be used once per day.

The Lover
The Lover is one of the Thief's Charges and her season is Sun's Dawn. Those born under the sign of the Lover are graceful and passionate.
Mooncalf: Permanent +1 bonus to Dexterity score.
Lover's Kiss: Upon a successful melee touch attack, your target is paralyzed for 5 rounds + ¼ your level(minimum 0), but you become fatigued. This spell-like ability can only be used once per day.

The Mage
The Mage is a Guardian Constellation whose Season is Rain's Hand when magicka was first used by men. His Charges are the Apprentice, the Atronach, and the Ritual. Those born under the Mage have more magicka and talent for all kinds of spellcasting, but are often arrogant and absent-minded.
Fay: +1 bonus spell slot equal to half your level(minimum 1, maximum 9) or lower.

The Ritual
The Ritual is one of the Mage's Charges and its Season is Morning Star. Those born under this sign have a variety of abilities depending on the aspects of the moons and the Divines.
Blessed Word: You can, as a standard action, cause one undead target within 30 feet of you to flee, as if panicked. Undead receive a Will save to negate the effect. The DC for this Will save is equal to 10 + 1/2 your level(minimum 1) + your Charisma modifier. Undead that fail their save flee for 5 rounds. Intelligent undead receive a new saving throw each round to end the effect. This spell-like ability can only be used once per day.
Blessed Touch: As Blessed Word, but upon a successful melee touch attack instead of a 30-foot range.
Mara's Gift: Cure 2d8 points of damage + 1 point per level. This spell-like ability can only be used once per day, and can only affect its user.

The Serpent
The Serpent wanders about in the sky and has no Season, though its motions are predictable to a degree. No characteristics are common to all who are born under the sign of the Serpent. Those born under this sign are the most blessed and the most cursed.
Star Curse: On a successful melee touch attack, the target is considered afflicted with a short-lived but dangerous poison. The target takes 1d6 points of poison damage +1 per half your level(minimum 1) every round for 3 rounds +1 per ¼ your level(minimum 0), including the turn they were afflicted, while the user takes 1d4 damage +1 per half your level(minimum 1) for 2 rounds +1 per ¼ your level(minimum 0). This spell-like ability can only be used once per day.

The Shadow
The Shadow's Season is Second Seed. The Shadow grants those born under her sign the ability to hide in shadows.
Moonshadow: Become invisible for one minute per level(minimum 1). This spell-like ability can only be used once per day, and only affects its user.

The Steed
The Steed is one of the Warrior's Charges, and her Season is Mid Year. Those born under the sign of the Steed are impatient and always hurrying from one place to another.
Charioteer: +10 move speed on land.

The Thief
The Thief is the last Guardian Constellation, and her Season is the darkest month of Evening Star. Her Charges are the Lover, the Shadow, and the Tower. Those born under the sign of the Thief are not typically thieves, though they take risks more often and only rarely come to harm. They will run out of luck eventually, however, and rarely live as long as those born under other signs.
Akaviri Danger-Sense: Permanent +1 per ¼ level(minimum 1) dodge bonus to AC.

The Tower
The Tower is one of the Thief's Charges and its Season is Frostfall. Those born under the sign of the Tower have a knack for finding gold and can open locks of all kinds.
Beggar's Nose: Detect any animals, keys, or magical items within 60 feet of you. 1st round, presence or absence of these things. 2nd round, number of these things and power of the most potent(whether that be a powerful animal or magical item). 3rd round, the strength and location of each of these things. If the items or animals are in line of sight, you can make Knowledge (arcana) skill checks to determine the school of magic involved in each. (Make one check per aura: DC 15 + spell level, or 15 + 1/2 level for a non-spell effect.) If the aura emanates from a magic item, you can attempt to identify its properties (see Spellcraft).
Tower Key: Immediately open any simple, average, or good lock. This effect can even open arcane locks. This spell-like ability can only be used once per day.

The Warrior
The Warrior is the first Guardian Constellation and he protects his charges during their Seasons. The Warrior's own season is Last Seed when his Strength is needed for the harvest. His Charges are the Lady, the Steed, and the Lord. Those born under the sign of the Warrior are skilled with weapons of all kinds, but prone to short tempers.
Warwyrd: +1 per ¼ level(minimum 1) to all attack rolls made with manufactured and natural weapons and unarmed attacks. This bonus does not apply to spells, spell-like abilities, or incorporeal or summoned weapons.

As always, feel free to use any of this content in any campaigns you run!

Disclaimers:
If I come up with anymore ideas, or certain aspects of it need to be changed for balance purposes, this submission itself will be altered. Changes or additions will be marked as such, for convenience. Any lore or mechanics-based disputes, questions, or otherwise may be presented to me via notes. I check notes regularly, but I will likely miss many questions or disputes if they are in the comments section below this submission.
I do not own any of this content, nor do I claim to own any of this content. All Pathfinder content utilized in this submission is property of Paizo Publishing, though some 3rd party content is also used throughout.
All Elder Scrolls-related content utilized in this project is property of Bethesda Softworks.
Everything I post related to ESPA should be considered nothing more than 3rd party content, and highly unofficial in all ways.
Elder Scrolls lore utilized in this project is promised to be as accurate as possible, but many adaptations and ports I must create for this project to operate properly must be fabricated from little to no mechanical/statistical information, by no fault of Paizo Publishing or Bethesda Softworks.
Main sources of lore and mechanical/statistical information include Unofficial Elder Scrolls Pages and Elder Scrolls Wikia.
I'm just a simple fan of both Pathfinder and Elder Scrolls, and wished to put my creative skills to the test combining them together.