Notice this. 25mm X 1.414 = 35mm. 35mm X 1.414 == 50mm. And 1.414 is the square root of 2. Long story short, this means that a 25mm lens covers twice the picture area of a 35mm; which in turn covers twice the area of a 50mm. So these three focal lengths make a nice system together, well spaced--not too close, not too far apart. And if we wanted to go wider, the next one to add at the wide end would be 17.5mm (in practice, 18mm is close enough). The next one to add at the long end would be 70mm (for Leica M, make it 75mm).
How to fit a 28mm into a similar system? Like this: 20-28-40 makes a good start. The next step would be add a 56mm. So in a Nikon system, we could use the 55mm MicroNikkor. And after that, the 85mm would be a fairly good fit into the square root of two progression. In the Leica system, we don't have a 55/56, and 50 is a bit too close to 40; while 75 is too long to fit the progression. Still, one could live with 21-28-40-75.