1. The city center sits at the center (because, duh).
2. The center is a basic shape, like a circle or square (for visual simplicity).
3. The center is zoomed in (because that area is always congested with lines).
4. All lines must run vertical, horizontal, or at 45-degree angles (again, for visual simplicity).
5. Their angles should be smooth (to feel more familiar, city to city).
6. Their colors and connection iconography are standardized (duh again).
7. All text must be listed in local and Latin lettering (for the tourists, aka all of us).
The subtext to subway remapping projects is often "London basically got this right 80 years ago, deal with it."— so his version of The Underground, above, is interesting food for thought.