Most any aux buildings could be prototype somewhere. I would at least have a power plant next to the roundhouse, to supply steam for heat and to keep locomotive boilers hot and pressurized while not in use. On the other side of the turntable should be fueling, water, and sand facilities. Maybe a heavy repair shop, separate from, but right next to the roundhouse. Specialty shops like a paint shed could be there too. Crew quarters and office buildings could be right there as well. A small loco, like an 0-4-0 saddle tanker, could be assigned there to move cold locos around. Lots of ideas out there, most of them could be found on some railroad somewhere. Don't forget piles of parts and junk, railroads didn't throw anything out they could find a use for someday.
They tell me to act my age. I don't know how to do that. I've never been this old before.