Extension of the feature in several variants where monster spawns are concentrated on the upstairs during the ascension run: occasionally, instead of spawning a monster, generate an organized group which is trying to get the Amulet, usually made up of sapient monsters (such as a group of soldiers, or a few priests of the same god, or some elves led by a couple elf-lords). Possibly, this only happens after you get back out of Gehennom and are again visible to the other gods.

Make monster summoning less common, but make group spawns more common. (Not necessarily meaning group spawns caused by monster summoning.) In particular, when a monster that is a lord or prince to its kind spawns, it should always spawn in some associated retinue along with it.

Assuming monster casting has been expanded somehow to include ally buffing: an orc shaman can generate with a group of other orcs.

Monsters that spawn in groups are regarded by the monster-generation function as having a higher difficulty than they actually do.

Remove your experience level as a factor from the logic that determines how large a newly generated monster group will be. Instead, scale the group size based on depth. (And to take this further, groups created by magical means such as create monster should be independent of both factors).

Monsters in groups should always spawn with the same affiliation (peaceful/hostile).