A few ideas for Diagetic UI
Billboard: Some kind of building with a procedural texture would be nice. When opening its UI you could write text which would be rendered on its texture. Imagine basically a flat billboard lying on the ground. Perhaps even have it switchable by green and red signals

Waypoint with Text: This would be some sort of floating icon with text above it, or maybe an arrow pointing to a location.
Arrows and Path Tiles: I'm not sure how easily new ground tiles can be added, but here custom textured concrete would work well.