This is a rather specific suggestion, but it could be blended into a more general concept of disguises. It's possible right now to assume an identity, but right now you can only invent an alias, and you can only assume the identity of a member of your own species. But it would be neat if you could attempt to assume the identity of an existing historical figure (like pretending to be the king of a civilization), or a member of a different species (or a different caste of your own species).
Of course, imitating a different species would have to have some kind of mechanism to determine how similar two castes/species looked. A female human could plausibly imitate a male human, or an elf, but would have a harder time imitating a male dwarf, and would have little hope in imitating an elephant man, or an elephant...but an elephant man might be able to imitate an elephant, at least to someone who wasn't very good at observation.
I'm not really sure how this could be done. Possibly using creature tags, like [RESEMBLES:CREATURE:CASTE:amount], and [RESEMBLES_CLASS:CREATURE_CLASS:amount] for more general categories. For example, a human female could have:
[RESEMBLES:HUMAN:MALE:50]
[RESEMBLES:ELF:FEMALE:70]
[RESEMBLES:ELF:MALE:60]
[RESEMBLES:DWARF:FEMALE:60]
[RESEMBLES:DWARF:MALE:20]
[RESEMBLES_CLASS:HUMANOID:10]
This wouldn't necessarily work both ways; it would just be a way of abstracting how easy it was to disguise yourself as a particular species. Creature size range could also figure into it.
The exact means of disguise should probably be abstracted. While it might be possible to implement something like face paints and makeup, this would be a lot of extra work for not a huge amount of gameplay payoff. Maybe some articles of clothing like cloaks and masks could have some extra tag that lets them boost one's disguise ability (while also making the bluff target more likely to get suspicious if you're wearing them).