Regarding https://fritzing.org/fritzings-graphic-standards/, I think there are a few extra useful things to add:

• Schematic: Stroke width of part outlines (for parts that are just outlined boxes)
• Schematic: Corner rounding and join mode of part outlines (minor but would be nice to have consistency, even if it's just to explicitly specify no rounding or whatever)
• Schematic: Mention that pins should be given terminals at their endpoints (minor)
• Schematic: Orientation for rotated text (in particular, if 45 and 90 degree labels should have baseline on left or on right)
• Schematic: If part outlines should be adjusted to place stroke inside, centered on, or outside of the 0.1" grid (minor, and arbitrary, just another one for consistency).
• Schematic: Offset from connector pins to their labels + numbers (for applicable parts)
• PCB: Recommended silkscreen line stroke width
• PCB: Enhance the description of PTH hole sizes to stress importance of checking actual part pin shapes, diameters, and datasheet drill hole size recommendations first (+ adding clearance; here is a decentish summary).

Each of those items is a metric that seems commonly used, and also often inconsistent among parts. From what I've seen anyways.

That's all I can think of right now although I feel like I'm forgetting a few. Will add more in comments if I think of any.