Your neck becomes perfectly cylindrical, and the bones inside disappear. Despite this, you have no issue with locomotion or keeping your head up, and your spinal column or breathing is not affected, but you can now turn your head 360 degree around. However, you take x2 damage from sneak attacks with blunt weapons, as getting hit in the head causes it to whiplash violently and be easily battered around.
 Wings. They start vestigial and make it hard to wear chest armor if it isn't built for it. The type of wing depends on your race and character- humans and most mammalian species get feathered wings. Kobolds and lizard people get scaly dragon like wings. Subterranean races like Orcs and really edgy people get bat wings, and so on.
Your wings don't work at first. However, each time you are hit with a powerful healing spell like regeneration, or restore at least 4 stat points when you do not have any damage stats; some of the magic from that may flow into your developing wings. Additionally, whenever you receive a beneficial mutation, your wings have a ½ chance to develop further instead of gaining a new mutation.
After your wings develop once, you can now flap them hard enough to create a little wind and reduce falling damage as though you fell 20 ft less.
After developing your wings twice, you can glide and slow your fall, meaning you cannot be harmed by falling damage as long as your wings are not bound and you are consciousness when you fall.
After your wings develop the third time, you can now fly. Flying is tiring but very useful- you can only fly while unencumbered and cannot carry anyone else unless they're a very small creature depending on your strength score. Your wings are still very sensitive and will be easily mangled by arrows or direct attacks. Be careful.
If you someone develop your wings 8, 10, or even 15 times or more your wings just keep getting stronger; letting you wear armor while flying, carrying larger beings up to your size, and even letting your create a few magical gusts of wind from wing flaps once or twice per day.
 Babification. Your body shrinks, your eyes get bigger, you get cuter and more harmless at the same time. Drop your Strength score by half, but increase your Charisma modifier by +1 whenever you're trying to look cute, innocent, or need protection. If you're already of a small race or are an old person, this mutation produces some very weird or off putting effects. You're still an adult, physically and mentally, just have a shorter statue and higher pitched, squeaky voice.
 The hotter your flesh gets, the closer it becomes to clay. Your skin turns more brown or tanned, becomes softer and malleable, and sweats slick. Permanent alterations can be made with the aid of a hot stick of metal and taking 1 damage per minor alteration. This offers no protection from heat- If you die from fire damage, you melt.
 The colder your flesh gets, the closer it becomes to steel. Your skin turns more blue and gray, becomes harder and unyielding, and condenses water on its surface. You gain +1 AC for a number of rounds equal to the damage you take from any cold spell, attack, or ice-dragon breath attack. Multiple sources stack, up to +3 AC from your steel cold flesh. This offers no protection from the damage of ice or cold. You also get a bonus +1 AC if you're in cold weather.
 Made out of horn or keratin, your face grows a strange symbol. It will be shaped with rings, angles, lines, and curves around your eyes and nose; never blocking your vision but giving you a strange facial appearance. It blocks your ability to wear masks or the vast majority of helmets, but you can use the symbol as a holy symbol once per day; channeling your wrath towards the undead.
If you know a magical symbol spell, there is a 1 in 6 chance that the symbol on your face matches one of the spells, and the daily power of this mutation is instead a single use of that spell channeled through your face; the symbol glowing red hot.
 A streak of random color beings to appear on your body. Starting at the tip of your hair and moving down through your skin, one eye that it crosses through, and down to your groin. This streak of color is permanent but has no special effects beyond giving you an exotic look.
 Pick one of your hands at random. This hand loses all fingers, becomes green and slimy, and grows 4 prehensile tentacles instead. The tentacles can be used to hold regular objects, but also grant +1 to grapple checks as the suckers help hold your foes in place.
 You grow a dorsal fin. Makes wearing most armor impossible and tight fitting clothes a chore. When swimming, you can pretend to be a shark to scare people and automatically succeed against saves to be drowned in murky dark water; the fin helps stabilize you but not necessarily make you swim any better or hold your breath longer.
 Scissor-Hands. Your hand are replaced with long blades, involuntary snapping claws, evil little snakes, etc. Makes it hard to hold anything without cutting it. This change isn't permanent, and these offensive fingers slowly fall off over the course of the next 3 days, each one becoming a +1 magic weapon or creature with appropriate stats.
 Your lips swell up to 3x the size. They pop loudly if punctured, they take an hour or so to reinflate. Treat your kisses as having the magic of a 1st level Princess.
 You gain a swelling green mark on your skin under your left armpit. Beings from other realms start to take an interest in you.
 Your head swells up. You gain +1 to your Intelligence modifier, but get -1 to all saves and AC from being off balance.
 Your body becomes somewhat conductive to elemental energies- your skin develops a waxy sheen. This is mostly useful as a parlor trick- hold a candle up to your foot and cough out some smoke. If you are struck by an elemental spell or dragon-breath, there is a 1 in 6 chance you can conduct that energy as a bolt or blast centered around yourself- dealing 1d4 damage per dice worth of damage that struck you. You still take the normal damage from the blast, but if the damage would kill you increase the damage of your conductive force by +1d4 and then die.
 Your face becomes itchy and stiff. Your face is now a “mask” and can be knocked off your head by a successful grab or by getting hit in the back of the head with a blunt weapon. When your mask is off, you cannot see or speak, but you can still breathe. Additionally, your 'mask' face can be exchanged with other people's mask faces, or with specially made magical masks made from faces, such as ones purchased from a Goblin Market. By using these face-masks, you can create disguises or even gain new abilities depending on the face, but your true face has become a simple mask that could be lost or stolen.
 Several bits of your flesh “bubble up” and then fall out of your body. These lumps harden and levitate, floating around you. You don't feel any pain from these flesh lumps but they are still “alive” and can heal themselves when damaged- but your body is weakened from the loss of tissue. You lose -2d6 hit points and gain +3 AC from the levitating protective body parts. If these lumps are destroyed or trapped, then you will lose the bonus AC from them and cannot recover the hit points.
 A prehistoric beast grows from your chest! It looks like a baby version of itself, its head poking out from a strange egg-shaped hole in your sternum. It can bite for 1d4 damage, and will loudly cry out for food if not fed, thus making you lose any stealth checks you were attempting. After a year, it will grow larger and fall out of your body, becoming a loyal dinosaur pet. Roll a 1d6+14 on the Random Animal Table to determine what prehistoric beast it is.
 You gain a weak spot. Randomly determine a spot on your body, it is NOT mirrored on both sides, so your weak spot could be just behind and under your left knee, for example. This weak spot is the size of a penny. If it is directly hit by a bladed weapon, arrow, strong punch, or other damaging attack you crumple in pain and drop to 1 Hit Point instantly. The weak spot does not look especially different from the rest of your body, and only you know its location.
 One of your legs becomes covered in a dark black insectoid chitin. You gain +1 AC, and this leg can also release a foul smelling gas once per day, which causes a morale check for predatory animals and other creatures sensitive to smell.
 You grow a tail. If you're a skin covered human, orc, elf, and so forth your tail is a gross rat like tail. If you're a furred creature, your tail will be furry instead, and if you already had a tail it doubles in length and size, granting you a +1 to your Dexterity modifier. Birds or insects instead get their tail feathers or thorax expanded in size and strength, improving their abilities to fly or shoot webs or improving your poison for scorpion stinger, and so on.