At some point during the concept phase, I came up with the idea to retexture the security guards to resemble military police guards who you often see guarding secret government desert facilities in media. Black Mesa is partially inspired by such real life facilities as in Los Alamos, New Mexico. Thus, "Topside Barney" was created.
This also gave me the idea to have the mod take place primarily in low security areas of Black Mesa close to the surface. Additionally, there's a throwaway line at the beginning of the base game during the tram ride made by the onboard announcer referencing job openings for "low clearance security". Part of the fun of developing this mod for me is the opportunity to fill in areas of Half-Life's lore that were left unexplained in the base game. There have been plenty of creators in the Half-Life community who have come up with good ideas for this but I like being able to apply my own unique creative spin on it.
Later on, when I decided to include office areas, I realised that lab coat wearing scientists didn't make much sense to be walking around in offices instead of labs, so I also retextured them to look more like they're wearing business suits. Unfortunately, their coats still look like lab coats which is down to me not really understanding how to edit Half-Life models beyond their textures.
Aside from custom models, another thing I chose to tackle was custom textures. Half-Life's base textures are very versatile but I found them lacking for the kind of visuals I really wanted in my mod. For the newer topside facilities in particular, I was inspired by the clean, minimalist and modernist style of another game called Mirror's Edge. This is one of my personal favourite games and its aesthetics largely contribute to that.