For the trait-specific improvements, I have been going back and forth on a couple. Researching the historical purposes of Guild Halls there seems to be some discrepancy. From Wikipedia, "A guildhall, also known as a 'guild hall' or 'guild house', is a historical building originally used for tax collecting ..."
So, one idea is to make it a unique improvement for Commercial Civs as an add-on to marketplaces. They would then have a Customs House SW that requires x number of Guild Halls. The other idea, and how I have seen other mods implement Guild Halls, would have them as the unique improvement for Industrious Civs serving more as a "craftsmen" guild. I could then have National Labor Union as the SW.
My original idea for Industrious Civs was the Smithy (forge) for the unique improvement and Ironworks as the SW. However, I am leaning towards having the Smithy act as a "barracks" for non-Miliray Civs. Military Civs would be able to build Barracks in every city. Whereas, non-military Civs would only be able to build a Smithy in a city with Iron in the radius, Stables in a city with horses, and later the War College in cities with Saltpeter.
If I have Smithy as an improvement for everyone, and the Guild Hall function as a "tax collector office" like Civilopedia states, then I don't know what to have for a unique improvement for Industrious Civs.