GUILDS:
Each guild hall build provides a bonus for each hall of the same guild across the realm or the world. The cost to build each guild hall increases proportionally to the number of halls of this guild in the world, because of logistics problems. The guild HQ is a world wonder counts as a guid hall and substitutes the guil hall, it has the same power of the hall, but where is writed realm read world, plus the special abilities below.
Iluminnati Guild:
Iluminati Guild hall (replaces the money changer)
Each guild hall reduces the amount of gold needed to buy production, mercenaries or pay for spy actions.
+ 30% to gold produced in the city.
Can build Spy.
+ 1 XP to spy produced for each Guild Hall in your realm.
Iluminati Guild HQ
When a city with Bankspy guilhall rebels it can join your empire.
Every time a player buy production, mercenaries or use a spy you gain gold.
Steampunk Guild:
Steampunk guild hall: (replaces siege workshop)
Produces Siege machines and gunpowder units
Rise the military production for each guild hall in your realm.
Generates a unhappy citzen for each temple in the city.
Cannot be build in a holy city.
Mage and disciples cannot be build in the city.
Steampunk HQ:
Workshops in the Grand Cross procuces two more production.
Choping produces more production, but forests and jungles don't grow naturally.
Cannot adopt Nature Guardian.
I like the idea of having Guild Halls that increase in power proportional to the amount of guild halls in your empire.
I think this would be a good way to improve the almost completely useless civic "guilds". You would only be able to build guildhalls with that civic on, and switching away from it would erase the halls, just like switching away from a religion will erase its heroes. Only 1 kind of guild should be buildable in each city, forcing you to make the choice between building a lot of one kind for a grand effect, or building fewer but placing them in a more tactical way, so that the smaller bonuses would amount for more in those particular cities(Focus science in the city with the Crown of Acarien, and gold in the city with Bazaar of Mammon, for example).
Of the particular guildhalls you are suggesting: I think the Illuminati Guild would be better if it was split up into the Thieves Guild and a Merchant's Guild. I'm not sure a Steampunk guild is in tune with the lore of Fall from Heaven.
Here is my suggestions for possible guildhalls:
Thieves Guild: As it is now, perhaps a bit more powerful. It might be too restrictive for the player if he was forced to use the Guilds civic to build Shadows, so perhaps allow Shadows without the guild, but make it give production boost or xp boost to them. Perhaps also allow a simple "Thief" unit that can be deployed in enemy cities to drain small amounts of gold. More thieves guilds might allow your civilization to have more Shadows(they are national units now, right?), or simple give them a larger xp-boost or let them start with the Commando promotion, something like that.
Merchant's Guild: +1 trade route in the city for each Merchant's Guild existing in your empire. Perhaps also a +5% to cash for each guildhall. Allows 3 merchant specialists.
Mage Guild: +1 xp to newly trained arcane units for each mage guildhall in existence. Perhaps also a +5% to science for each guildhall. Allows 3 sage specialists.
Fighters Guild: +1xp to newly trained military per guildhall. +5% military production for each guildhall in existence. +4 Great Commander points.
Artists' Guild: +1 happy face for each Artist guild in existence. +10% culture for each guildhall. Allows 3 bard specialists.
Craftsmens' Guild:+1 unhealthiness for each guildhall in existence. +5% hammers for each guildhall in existence. Allows 3 engineer specialists.
The guilds are somewhat similar in function to the Orbital Facilities of Alpha Centauri, being quite weak alone but stronger the more that are built. I think the cumulative effect of guilds should be capped, though, perhaps at 5 guildhalls.
It would make for interesting decisions when a player builds guildhalls, as he will have to choose between building a few kinds of guilds to great effect, or specializing individual cities more at the cost of smaller effect.