While the machinists' guild should be in a good position at this point in the story, it's variable depending on whether the player has completed the MCH questline or not. From the way they normally handle optional sidequests, I'd be surprised if they pretend it's happened regardless - although we could potentially still get Stephanivien appearing, disconnected from the MCH story, if we need to deal with Skysteel Manufactory as part of the rebuilding. I just wouldn't expect to hear much else of how the guild is going, or see any of the other characters.

If we have recurring characters from elsewhere, I would expect to be dealing with characters who've either been introduced in the MSQ or are standing around the town. I'm hoping Emmanellain and Francel will be involved, and if we're revisiting the Diadem then Aurvael might actually get to do stuff. Edmont will probably make an appearance.

Though if we're dealing with new characters, we're certainly overdue for hearing more from the High House best known for its architectural skill...?

Well, that and being the designated villains in any feud we're aware of between the houses.