I want to apply the specialities concept to the Literacy skill in the Dawnlands (my Mythras iron-age central Asian-inflected setting), but without simply having it be a repeat of the specialities of the Language skill. A simple repeat of the same specialities would just turn Literacy into a skill tax imposed on PCs. I also think it's pretty boring.
I also think we need to avoid the obvious extension of it, which is to separate the ability to interpret and decipher writing in a particular form into speech. I initially made this error and had five different alphabets, syllabaries, abugidas and pictograms, which Literacy would let you turn into something you then needed a Language skill to make sense of. I think this would increase referee cognitive load in planning and preparation, without adding much to the game.
You, my well-educated audience, may have already encountered the idea of "literacies" in contemporary educational theory. This is often used in the context of explaining various digital media competencies, but I think elements of this can be projected backwards in time, and laterally, for our purpose, to make the Literacy skill interesting and fun. To tip my hand, I want to expand the Literacy to cover a variety of hermeneutic practices, of which reading plain text on a page is only one example. Literacy now becomes the skill of interpreting symbol sets other than speech. I do want to be careful not to step too far into the domains of other skills and replacing the need for Customs, Culture, Lore or Art, but I think there are a few pieces that could fall under Literacy or one of these skills that we ought to bring under the Literacy skill.
NB: Along with allowing you to decipher the types of texts below, I think that in many cases Literacy should also cover producing examples of them.
Here's a brief list of ideas of interpretive practices that might be important to someone in a fantastical pseudo-ancient or pseudo-medieval setting.
1) Reading out loud
2) Codes and ciphers
3) Dreams, omens, oracles
4) Technical, mathematical, and scientific jargon and diagrams
5) Financial and legal records and accounts
6) Reading silently
7) Magical writing (or this may be a subset of #4)
8) Maps & calendars
A brief justification for each of these as ideas:
Reading out loud and reading silently are separate developments historically, as weird as it may seem to a modern person trained in doing both from a relatively young age. It seems like in the Western world, reading silently emerges shortly after monasticism, as part of the contemplative practices of monks. Until that point, so far as we can tell, people mostly read things aloud, even when they were reading for themselves. Breaking them up as specialties is a minor but fun idea with the effect of estranging the setting in a subtle way for players.
Codes and ciphers represents the ability to encipher and decipher texts written in codes and ciphers. It's handy and it doesn't cleanly fall under any other skill unless you make up a Lore speciality covering it. If you have "thieves guilds" or the like, you might want to make up a separate speciality for their specific codes, but I think the narrower this speciality, the less useful it is.
Dreams, omens, and oracles are in the representations we have from the ancient world almost always vague, riddle-like things that require expert interpretation, and dramatically much can turn on the ambiguous possibilities of an oracle or omen. I think this should also cover things like astrological charts, hexagrams from the I Ching, and the markings on the intestines of sheep. I think this is, like literacy in codes and ciphers, rapidly becomes less important or useful the more narrow it is (i.e. just interpreting dreams or just interpreting sheep intestines or just looking at chickens pecking grain out of a grid).
If you've ever tried to read an old mathematical or technical manuscript, you probably understand why this is distinct from one's familiarity with the scientific concept under discussion, or one's ability to read the plain text of the manuscript. For that matter, if you've ever seen two people quibble over what a blueprint means, you've probably had the same experience. Diagrams can be surprisingly ambiguous, especially if it's stylised so that particular design choices are intended to cover specific information rather than serve as a picture. It's also less relevant in an ancient or medieval setting, but I think reading graphs probably falls at least partly under this speciality as well. Whether you want to make a "high-falutin' writin'" speciality that combines this with the no doubt extremely similar problems of interpreting magical writings is your preference. I would separate them into two specialities mainly as a matter of personal taste.
Financial records and accounts remain a specialised form of literacy with entire certified professions dedicated to them (accountants, stockbrokers, etc.). Understanding them is distinct from mathematical knowledge per se (which I think is properly one or more Lore specialities). Historically, this type of writing precedes the others - records of debts and receipts are the oldest writing we can find evidence of. Legal records and documents, which are often tax records of some sort historically, are similarly obtuse and impenetrable even if one has a rough and ready sense of what the actual law applying to a situation is. You may want to roll these under the Commerce and Bureaucracy skill, respectively. Mythras doesn't have a forgery skill, and allowing this as a speciality allows you to make a forger, which I think is something PCs want to do often enough that it's worth having a special skill covering.
Maps and calendars are really two different types of literacy in real life (interpreting abstract spatial representations and abstract temporal relations), and understanding them were specialised skills historically. Thucydides found calendars in contemporary Athens so confusing that he simply invented his own method of tracking time in his historical work. How to calculate the exact date of Easter is a perennial dispute amongst the Christian sects even now. I'm not sure either kind of literacy is quite useful enough to be worth a speciality on its own, but together they're fairly handy, especially since having them as a Literacy speciality should allow a PC to produce them.
NB: I considering reading maps quite different than the Navigation skill, since the later covers going to places, and maps do all sorts of things other than guide you somewhere (here's a neat one that's useless for navigation).
Some of these might reasonably be Lore specialities instead of Literacy specialities. But, I think one thing to bear in mind if one is using the specialities system is that getting more than 5 specialities in a particular skill is a challenge because of the difficulty of acquiring skill ratings above 100%. So loading some potential Lore capabilities onto Literacy means that characters don't have to sacrifice one of their Lore specialities to get ahold of them, and can instead raise their Literacy skill (which is often surprisingly low).
Other than the ones listed above, I'm open to suggestions for other Literacy specialities.