Honestly - when it comes to fluff the biggest gap is with the Vermin Swarm - finding out how to fit them into the picture, flush out their place in the world, and figure out their linguistic/cultural roots. As it has been asked - Are they Roman or Byzantine? If so, which era? both of those span almost 2000 years of history.
Linguistic and cultural roots: If they came into being during the fifth age, as suggest by the tapestry, they may not have any other linguistic roots than what the local language was at the time. If that was latin then latin it is, which is good as I do not speak greek. As they were driven away and went into a underground diaspora they took up the languages of their surface neighbors. Latin remains the "language of power" as it is used by politicians and learned rats and used in diplomatic text between clans. It is revered so much that is is common for those who can not speak latin but have come to power to pretend speak latin in front of the lower classes with phrases like "hokus pokus". The lower classes speak a lot of different languages mixed with a vulgar latin. As they have many slaves pigeon and creole languages are common among the slaves and slavedrivers. They use latin writing for laws and important documents but have a second writing language that is based on characters. The character is written whit scratch strokes that are wider at the base and then becomes narrower so it can be scribbled with a claw or a brush. Each character has the meaning of a word and the language is simple and can be used without knowing the pronunciation of the word. It is usually used for signs and commandments but lack the proper grammar to form proper sentences in most languages. With these characters the swarm can communicate over verbal linguistic differences in their polyglot society. They complement their verbal language with smells and onomatopoeic sounds which makes their speech sound primitive to most humans. They are also more animalistic in their body language than humans and show their neck in submission rather than raise their hands.
Are they Roman or Byzantine? If so, which era? both of those span almost 2000 years of history: They have a voting system for their leaders according to the 9th Scroll and yet the Byzantine Empire suits their assassin riddled culture, so I should say a mix it. The first triumvirate contains voting and a republic setting with infighting and a constant eroding of the old republic. Mix that with Byzantine Empire under the Angelos dynasty that gives rebellion and a combination of fight against corruption and tyranny. The latter is also close in time to the viking age and the age of chivalry.
Finding out how to fit them into the picture, flush out their place in the world: They are the constant threat to the great civilizations. They do not want to destroy the world like the daemons, nor do they reflect our sins and worst behavior like the dark gods. They want to usurp the surface world and replace its population and continue what has been built and making it better in their view. Where there is a great civilization there is a swarm underground that speaks the language of it citizens and plots for its capture. They envy the surface livers and feel like they do not appreciate what they have and that they do not reach their true potential of what the surface can give them as they have become complacent. The fear of advancements in technology, magic, and selective breeding has chained the surface dwellers to mediocracy.
The swarm was given Avras to build upon what humans had created but were to conservative and blind to perfect their creation. They are opportunist that copy what other have created and makes it "better". They long for the old metropolis but most have never seen a city on the surface and for them Avras is but a metaphor for a better world.
As they plot for a renewed surface supremacy they infight and scheme. Some populists claims that the upper classes forgets about the need of the lower classes in their dreams of Avras and the populist seeks to replace to old elite so that they can distribute the swarms wealth to all citizens. This leads to political roots and uprisings from time to time.
As opportunist they want for division and infighting and when it happens they strike and colonizes.
"As pretenders clash in the fields a rat usurps the throne" an Avras proverb about the dangers of animosity within the family.