Ability to have multiple options of how you want nichelings to be born. Animals like elephants and humans are like the current nichelings and usually only have one child at a time, but this isn’t a good adaptation in some niches. Animals like mice and fish usually have many babies at once. This could be added as a sandbox option, Eg:
Option 1: Usually litters of one, rarely twins, very rarely identical twins
Option 2: Usually twins, rarely litters of one, rarely triplets, rarely identical twins/triplets
Option 3: Usually triplets, rarely twins, rarely identical twins/triplets, rarely litters of one
Since lots of animals in the wild have large litters, this might be useful for playthroughs with nichelings in dangerous habitats or short lifespans, and would complement lots of other available sandbox options well. It would make the game more exciting and also offer roleplay opportunities as well as the ability to show survival of the fittest in a faster paced game.
Alternatively, this could be added as a gene in its own right, a bit like fertility.