To make truly alien cultures we need to accept that what we think is inalienable truth is just a coincidence of time and place. Roll it back all the way to monkeys in trousers, assume the fundamentals go no further and just pick the colour.
The following assumes essentially human. More alien people should be weirder. It is important to remind yourself that these are not people who are repressed, who need to be shown the way, be freed or educated. They are right, what they believe is true, you are weird.
- Weaponry is obscene and must be covered. Soldiers might fight naked, since they are basically sluts anyway. They whip off their burkas when trouble starts. Polearms upholstered in tasteful flowery linen.
- Energetic greeting. Headbutts, flailing, psychic probe, elaborate hand shakes. Harmless if both parties are good at it. Test etiquette or take damage as club.
- To refer to yourself is considered rude. Even doing weird robot talk like "this one" is quite tacky. Huge amount of linguistic acrobatics is required to ask for something "what a handsome sandwich that is" = "can i have that sandwich please". The people are considered very attentive to others' needs, but really they're just being pragmatic since no one can ask for anything.
- Displays of emotion are rude to the point of being non-existent. People smile and function normally, but showing sincere feeling is not done. Private lives might be rife with exuberant vented emotion, or it may bleed over into the bedroom. Possibility of traditional emotion, such as polite clapping, laughing at set intervals.
- A body part is considered highly sexual. 1-noses 2-ears 3-eyebrows 4-tongues 5-fingers 6-nails 7-elbows 8-backs 9-knees 10-toes 11-heels 12-rolltwice. 50/50 chance they will kept covered and subject to taboo and modesty, or exaggerated. Think prosthetics, makeup, revealing clothes. Thickly drawn on eyebrows, crotch extensions.
- Wearing shoes is disgusting. Poor people wear shoes for work, but usually just walk about with filthy, calloused feet. Rich people might have teams of servants picking clean the road ahead of them, taking forever to walk anywhere. Regular foot baths. Every house has at least a small muddy foot bath at the door.
- Noteworthy peculiarity in greeting. Kissing (consider: kissing foreheads, eyes, noses, ears. Asymmetrical, one kisses the nose, one the chin. Hierarchical implications). Exposure (lifting hats, raising/dropping trousers, flapping aside your cape. These actions may have lost the physical object they were associated with, thus lifting your cap is now an odd salute).
- Dancing/singing/comedy/theatre as routine parlour entertainment. Performed by attendees of meals or parties as matter of course. They may be improvised or may draw from a rich selection of traditional routines.
- Homogeneous. Everyone wears very similar outfits, or of similar materials, or a single colour. Maybe all clothes are made of wound rope or they all wear fezs and blue spectacles.
- Clothes optional. Nudity is casual and met with indifference. This may be limited to a gender or age group. Naked old ladies.
- Mutilation is common. Piercings, tattoos, fashionable dismemberment. Removal of nose/ear/eye.
- Everyone covered everything. One random body part is considered acceptable to be shown. Probably eyes or hands. Consider more exciting things than burkas, like ninjas or thickly wrapped togas where the people look like balls of wool with knitting needles in. This may be due to beliefs in modesty or just fashion.
- Pets. Common to use live animals as fashion accessories. This may be practical, like having a hawk to keep away pigeons, or purely display, like teaching a ferret to drape around your neck.
- The sexes are very different. Roll them completely separately. Very distinct look and expectations. In this culture cross-dressing will be possible and more common/prominent. Explore that possibility, consider implications.
- Hair is manipulated heavily. Held firm with animal blood or specific coloured mud. Total hairlessness as a sign of beauty. Kept huge with elaborate metal frames. Covered in dead animals. Replaced with cloth. Hair topiary, matted into shapely "hats".
- Nudity as social scale. The rich wear nothing while the poor dress in endless filthy layers. Nudity represents the access to warmth? The opposite may be true, where the rich wear clothes 'cos it's valuable, the poor are prevented. Like the the old Imperial Purple. Origins or traditions may be misty. Skin painting a distinct possibility.
- Families are communal under a patriarch/matriarch/location/street/date of birth. Blood relatives are not considered noteworthy.
- Marriage does not exist. Unions are like friendships, improvised, sprawling, personal
- All social engagement is official. Marriage ceremonies, friend ceremonies, enemy ceremonies. Most significant interactions must be played out within the confines of a relationship or else is considered illegal/immoral.
- Men/Women collect partners as a sign of fertility/wealth/power. Large families, easily enters hundred of grand children. What do the many single people of the dominant sex do? Underclass? Ruling class? Workers? Exiled? Killed? What are the interactions between families?
- Clans of extended families. Family name carries a lot of weight.
- Families have historical subservient families. Complicated interrelations and dependencies. Entire subclass of bureaucrats are needed to maintain this.
- Gender is determined by familial role. Between 3 and hundreds. Consider the multiple partner marriages: as the Xs marry large numbers of Ys the Xs left without the chance to marry become Zs. Circumcisions can change things, dress can be involved, soldiers might be a gender of their own, renouncing traditional gender states in favour of big soldier orgies.
- Stringent child screening. Babies aren't named until a certain test (physical or spiritual) is passed. Babies are left out for the wolves, the uneaten are taken in. Babies are submerged in chaos matter, causing them to be peculiar like everyone else.
- Pregnant women are sent to special pregnant towns, where everyone is pregnant. Run by mothers of miscarriages? Priests? Public pregnancy is likely either taboo, or obscene.
- Everyone is raised by the government. The king/pope/mayor is their father. Blood ties do not exist. Very patriotic.
- Flower wars. Ceremonial warfare is conducted. This will have different rules to all out war, set places, set traditions, set methods and purposes.
- Champions. Two individuals fight, or small ceremonial "armies" of 5 guys. With this sytem soldiers would be useless. Hero culture, stables of heroes being trained for war. Small handfuls of them. Live like sumo wrestlers. Maybe they sumo wrestle?
- Hill of the king. Wars end when the leader dies. Battles are mainly attempts to get to them. Kind of like a rugby match with two balls.
- Auction. Mercenaries long ago replaced standing armies. You won by having the best mercenaries the longest. In time people cut out the middle man and just competed on who could throw away the most money.
- No soldiers, only mobs. The leader raises a rabble many thousands strong and they rabble on over to their enemy.
- Coup only. Assassins are sent to infiltrate and kill en masse. Might be people or just politicians. Carries on until surrender, each side assassinating like mad.
- Monster hunters. The only military is grizzled monster hunters who track down the gnarliest cthonic beasts to unleash on their enemies.
- Seasonal. Wars are a matter of course. If you don't have an enemy you make one for a few months and go home.
- Sincerity. As long as what you do is done sincerely it is considered acceptable. If murder is done sincerely people may sincerely prevent you from doing so, but it is not evil.
- Hospitality is of primary importance. Welcome to homes, many traditions of protection or service.
- Peace. Anger, ambition, any conflict is considered extremely harmful. Those pushing too hard against the peace are evil. Retaliation to those breaking peace may be very hostile. Executions, trials, banishment.
- Ambition. Personal success is a priority. If you must harm others in you ascent it is okey as long as you win. Lots of business, war is common, retention of arms and power is desired.
- Kindness. The integrity of others' feelings is a priority. Politics is slow and gentle, fraught with frustrations.
- Secrets. Knowledge shared is knowledge halved. People are respected by the amount of knowledge they might have. Demonstrate it by dripping it out to apprentices. Gain apprentices in exchange for favours. Currency of secrets.
- Violence. Physical dominence is prime. If you can overpower an opponent you are correct. This may be personal power or the ability to accumilate those with it to your side.
- Piety. Religious sincerity, or just knowledge or trappings, is important. A good person is religious, perfectly observes tradition and so on.
- Individuality. Strive for uniqueness. Eccentricity loses its meaning among these people. Encourages taste makers, influencing others to imitate for the chance of mirrored originality.
- Academic. Being able to talk eloquently on a wide array of subjects. Everyone is learned, or at least capable of appearing so. Problems with half-knowledge being passed off. Pseudo-intellectuals.
- Renaissance men. Ability to perform many tasks is respected. Variety and vitality.
- Post modern irony. Roll again for the attitude that has been past. That attitude is parroted and mocked, a pale imitation. Some old people might still adhere to it, young people snear.