There's language civic in the mod, I think writing civic could be added too.
No Writing - Starting Civic - +1 unhappiness, -20% culture, -20% science, +20% espionage, +25% maintenance cost from distance to the palace, damages relations with civilizations using any other writing civic by 3 points
Cave Paintings - requires Petroglyphs tech, no anarchy time if changed from earlier civics - -10% culture, -10% science, +15% espionage, +20% maintenance cost from distance to the palace, damages relations with civilizations using any other writing civic by 2 points except Petroglyphs
Petroglyphs - requires Petroglyphs tech AND Carving tech, no anarchy time if changed from earlier civics - -5% culture, -10% science, +10% espionage, +20% maintenance cost from distance to the palace, damages relations with civilizations using any other writing civic by 2 points except Cave Paintings
Pictograms - requires Pictographs tech, no anarchy time if changed from earlier civics - +10% espionage, +15% maintenance cost from distanceto the palace, damages relations with civilizations using any other writing civic by 1 point
Ideograms - requires Ideograms tech, no anarchy time if changed from earlier civics - +10% culture, +10% science, +15% espionage, +10% maintenance cost from distance to the palace, damages relations with civilizations using any other writing civic by 1 point
Native Script - requires Writing tech, no anarchy time if changed from earlier civics - +20% culture, +20% science, +20% espionage, +5% maintenance cost for number of the cities, damages relations with ALL other civilizations by 1 point, improves relations with civilizations using Language Education civic by 1 point
Logographic - requires Scriptures tech, no anarchy time if changed from earlier civics, short anarchy if changed from Syllabic, Alphabetic, Abugidic, Abjadic, Featural, and later civics - +15% culture, +20% science, +25% espionage, damages relations with civilizations using any other writing civic by 1 point, improves relations with civilizations using Language Education civic by 1 point
Abjadic - requires Linguistics tech, no anarchy time if changed from earlier civics, short anarchy if changed from Syllabic, Alphabetic, Abugidic, Logographic, Featural, and later civics - +20% culture, +25% science, +15% espionage, damages relations with civilizations using any other writing civic by 1 point, improves relations with civilizations using Language Education civic by 1 point
Alphabetic - requires Alphabet tech, no anarchy time if changed from earlier civics, short anarchy if changed from Syllabic, Logographic, Abugidic, Abjadic, Featural, and later civics - +25% culture, +20% science, +15% espionage, damages relations with civilizations using any other writing civic by 1 point, improves relations with civilizations using Language Education civic by 1 point
Abugidic - requires Code of Laws tech, no anarchy time if changed from earlier civics, short anarchy if changed from Syllabic, Alphabetic, Logographic, Abjadic, Featural, and later civics - +15% culture, +25% science, +20% espionage, damages relations with civilizations using any other writing civic by 1 point, improves relations with civilizations using Language Education civic by 1 point
Syllabic - requires Paper tech, no anarchy time if changed from earlier civics, short anarchy if changed from Logographic, Alphabetic, Abugidic, Abjadic, Featural, and later civics - +20% culture, +15% science, +25% espionage, damages relations with civilizations using any other writing civic by 1 point, improves relations with civilizations using Language Education civic by 1 point
Featural - requires Education tech, no anarchy time if changed from earlier civics, short anarchy if changed from Syllabic, Alphabetic, Abugidic, Abjadic, Logographic, and later civics - +25% culture, +15% science, +20% espionage, damages relations with civilizations using any other writing civic by 1 point, improves relations with civilizations using Language Education civic by 1 point
Now it gets hard, but let's see...
Binary - requires <SomeFutureTech> - "Your Civilization uses pure information as writing system" - no anarchy time if changed from earlier civics - +40% culture, +40% science
Post Writing - requires <SomeFutureTech2> - "Your Civilization no longer has a need of writing system" - no anarchy time if changed from earlier civics - +50% culture, +50% science
Since there's a chunk of civics with similar bonuses (Logographic, Abjadic, Alphabetic, Abugidic, Syllabic, Featural), existing civilizations should have a preference for one of them.
Aboriginal - Alphabetic
Arabian - Abjadic
Assyrian - Logographic, Abjad, Syllabic
Australian - Alphabetic
Aztec - Pictograms, Ideograms, later Alphabetic
Babylonian - Logographic, will switch to Syllabic
Brazilian - Alphabetic
Byzantine - Alphabetic
Carthaginian - Abjadic
Celtic - Alphabetic
Chinese - Logographic
Denmark - Alphabetic
Dutch - Alphabetic
Egyptian - Logographic, Syllabic, Abjadic, Alphabetic ¯\_(ツ)_/¯
English - Alphabetic
Ethiopian - Abjadic, then switches to Abugidic if possible
French - Alphabetic
German - Alphabetic
Greek - Alphabetic
Hittite - Logographic or Syllabic
Holy Roman - Alphabetic
Incan - Native Script, Alphabetic
Indian - Abugidic
Iroquois - Alphabetic, Syllabic
Israeli - Abjadic
Japanese - Logograpic, then switches to Syllabic if possible
Jivaro - Alphabetic
Khmer - Abugidic
Korean - Logographic, then switches to Featural if possible
Malian - Abjadic, might switch to Alphabetic
Maori - Alphabetic
Mayan - Prefers Syllabic, might use Logographic, if pressured might switch to Alphabetic
Mongolian - Logographic, Syllabic, Abugidic, Alphabetic ¯\_(ツ)_/¯
Native American - Alphabetic
Ottoman - might switch freely between Alphabetic and Abjadic. Prefers Alphabetic, but should switch to Abjadic if religion is Islam
Persian - Logographic, Syllabic, Abjadic, Alphabetic ¯\_(ツ)_/¯
Polynesian - Native Script, Alphabetic
Portugal - Alphabetic
Roman - Alphabetic
Russian - Alphabetic
Siamese - Abugidic
Spanish - Alphabetic
Sumerian - Logographic, Syllabic
Tupi - Alphabetic
United States - Alphabetic
Viking - Alphabetic
Zulu - Alphabetic