I've been looking for a Guild where you could fully play the slavery, slaves and masters. Or even play ERP slavery. So it was fully structured Guild with slavery. Are there any such guilds on the EU server?
As far as I know there isn't any structured guild with slavery. There may be interested individuals, however. For that I would suggest you to join the OOC hub guilds and ask around on their forums and discords.
I'm afraid there aren't too many slavery guilds that I know of, especially of the ERP variety. You might check out the ESO ERP Community for more like-minded individuals, though I know nothing about that scene myself.