When you are “due” for a sacrifice and of a certain experience level, you gain the ability to sense altars from afar.

If you kill an enemy on a coaligned altar, you get a small alignment bonus and a message “Shimmering droplets of blood ascend into the realm of [deity]!” (Not specified what it should say if the dying monster doesn’t have blood.)

If in a variant that has Secespita, making the killing blow with that also decreases prayer timeout by a few turns.

When you convert a cross-aligned temple’s altar, there is a Cha/100 chance that you also convert the priest along with it, and they become coaligned rather than angry.

Priests cannot convert aligned altars, because they respect those other religions. However, they can convert Moloch’s altars, including the ones in Gehennom.

Several elaborations on a system for more reliable (and less tedious) gifts from altars:

1. Your god will always give you a gift on the first sacrifice from a given coaligned altar (not if attempting to convert an altar). But after that, they will not give you any more gifts on that altar no matter how much you sacrifice. (Other sacrifice effects will still happen as if you failed the die roll for a gift.) This eliminates all the tedium associated with altar camping for gifts (though not all altar camping in general), but is a significant change since getting one corpse to an altar is much easier than getting multiple ones, and streamlining the game to this extent may be undesirable.
2. As above, but the gift will only be given for the first monster whose difficulty is above a certain threshold (possibly partially dependent on depth, and also with some random factor). Thus, if you sacrifice and get no artifact, you know you have to try a harder monster.
3. The total difficulty of monsters sacrificed is accumulated; the first sacrifice to put you over some threshold (say, 30 cumulative difficulty) is guaranteed to give you a gift. The totaling may or may not be per-altar.
4. Your god has a set list of rewards, each of which is tied to some monster difficulty level. Some of these rewards are artifact gifts; others may be things like luck. You only get a gift when you sacrifice a monster of the requisite difficulty, and thereafter monsters of equal or lesser difficulty to that are ignored by the god for reward purposes (they may still be used for mollifying and decreasing prayer timeout). If you make a sacrifice that earns you multiple rewards at once, you either get them all at once or you get the biggest reward and the smaller ones on subsequent sacrifices. This would enable game developers to fix a number of artifact gifts (say, 3), by encoding that many rewards for them. This system might have problems in games with no deep usable altars where it is harder to get randomly generated difficult monsters.
5. Variation of the above: at the beginning of the game, three random monster difficulties are chosen (with some limiting on ranges). Sacrificing a monster of a difficulty above any of these thresholds automatically gives you a sacrifice gift and removes the highest remaining threshold you met.

A Priest in good standing with their god and standing on or near a coaligned altar sometimes has the ability to use a special spell or prayer to call a coaligned Angel to fight with them as a pet.

If you get crowned on an altar, it becomes a throne which will give you one guaranteed wish, and then has no effect other than to make you feel very comfortable there. Not specified what will happen if the altar is in a temple with a priest tending it.

There is some interaction you can do at an altar when you bring a zombie corpse there (not sacrificing it, since it will be too old) that sets the soul of the creature free, which will stop it from reviving.

You can get at most 1 sacrifice gift from any given altar. It can still be used for sacrifices and everything else, but you will never again get a gift from it.

Dropping items onto a Moloch-aligned altar (or only his high altar?) can curse them.

Converting an altar is made easier to do in terms of RNG (either increasing the odds or making the conversion guaranteed after some number of sacrifices), but in compensation for that, all attempted sacrifices cause the altar’s god to get angry at you and smite you, with the penalty dependent on your XL or level difficulty. They can do things like curse your items, give you negative protection, send minions to defend the altar, and blast you with lightning and disintegration if at a high enough level.

On deeper Dungeons of Doom levels, when the game randomly places an altar in a room without intending to create a temple, sometimes make the room into an abandoned temple.

Priest characters can sacrifice gold at a coaligned altar for identical benefits as they would get from donating to a priest. The altar does not have to be attended.

You can offer gold on a coaligned altar as well as giving it to a priest. This gives you a different, weaker, variety of benefits if there is no priest present (and the same ones if there is a priest present). When you do donate to a priest, they offer the gold themselves in a burst of light (which deletes the gold, preventing you from killing them or stealing to recoup it).

Make it easier to destroy non-high altars, perhaps by something as simple as digging them, but destroying an altar will cause its god to smite you hard. If you destroy a crossaligned altar, your god rewards you for it.

Some mechanic for Valkyries that involves escorting the spirits/ghosts of those who die in battle.

One take on this idea: you can chat to ghosts to tame them, and then they will follow you around as pets do. Then you can release their spirits by praying on an aligned altar. Doing this successfully gives you some sort of reward. However, hostile monsters will attack the ghosts you are escorting without provocation, making this not trivially easy.

Alternate ending where you can settle down in an abandoned temple or possibly just a random altar room and become the priest of the temple permanently. (Creating a bones file where you are now an aligned priest).

The more gold you pay to a priest, the stronger his god becomes. Newly generated priests of that god could get better equipment, or altars of that god might be harder to convert. In Moloch’s case, Gehennom gets slightly harder if you pour a lot of gold into his temple. This applies with money donated to your own god as well, but the effects are naturally less visible since you won’t go around fighting your own priests or converting your own altars.

Increase the rate at which annoyed gods will send down minions to defend their altars from being converted, since this rarely happens.

Abandoned temples can generate as a special room, or else a room containing a random unattended altar may be converted into an abandoned temple.

Aligned altars can generate in Gehennom, and standing on a coaligned altar in Gehennom is the only way to contact your god (via prayer or sacrifice). If the altar is crossaligned, you can try to convert it, but Moloch may seize control during the process and turn it into an altar to Moloch. Molochian altars in Gehennom are very difficult or outright impossible to convert.

Track the number of unattended altars generated in the Dungeons of Doom above, say, the Quest portal level. If zero have been generated yet, the chance of one generating on a new level in that range increases more and more until it’s a certainty on the final one.

Mimics can mimic other pieces of furniture like sinks, thrones, and altars. (They can already mimic doors and stairs.)

Unconsecrated or minor altars. They are either to some unnamed lesser god, or to no one in particular. They can be converted to your god like normal, but don’t have any penalty for kicking or attempting to engrave or anything while still unconsecrated.

A boulder that falls on top of an altar destroys that altar.

Priest characters get sanctuary in rooms containing unattended coaligned altars, even though they aren’t temples.

Dropping a lit candle on a coaligned altar gives you some message about your standing with your god and your ability to pray safely. The candle is consumed in the process, and possibly the beatitude matters (with a cursed candle plus an angry god ending in a fiery explosion). Some unspecified bad thing happens if you try this on a crossaligned altar.

Vomiting on an altar should offend its god, as should digging on it.

Dropping an item on an altar to Moloch may, in some cases, curse the item. Ideally this should not be exploitable, so things like a very small chance per drop of an item would not be good. Maybe tied to object ID number if that can’t be exploited.

The Oracle stands on a neutral altar, but killing her sets your luck to -10.

Special room “ruined church” that contains ghosts, pieces of glass scattered around the floor, maybe an altar, maybe a spellbook, and blank paper/scrolls of junk mail scattered around the floor.

Players can polymorph dungeon features by zapping a polymorph beam down at it, with some special cases:

• Gravestones can be created by polymorph and are more likely than other outcomes, but cannot be polymorphed into anything else. Trying to do so only polymorphs its epitaph.
• Polymorphing an altar angers that altar’s god and any attendant priest since you effectively destroyed their altar
• Fountains either cannot polymorph into anything besides headstones or have a very limited chance of becoming something else; this is to prevent the player from being able to generate a bunch of thrones from fountains.