I am proposing a combination of suggestions by other players and strategies for navigation that appear to have worked well on a smaller scale.
The first thing is the creation of eight new "main roads" starting at ''x = 0, z = 0'' in the Nether. There would be one each for North, South, East and West. And there would be one for each of the mid-directions - NE, SE, SW and NW.
The previous navigation of coloured wool paths leading to arches with main roads works very well for new players, and gives roads visual distinctiveness. So I'm suggesting that we colour each road with the seven colours of the rainbow plus one, starting at North and working clockwise. The combination of the colour wheel and the compass could work well as a mnemonic - if you see at-a-glance that you're on the green road, you can "count" clockwise from North/red and work out that you're on the South-East path. To fit the new rotationally symmetrical road system, I'm also proposing a new, square hub. To make sure it's navigable and difficult to get lost, I've added coloured posts with signs to match the corresponding archways - and there will be other landmarks such as a horse pen and a small family of mooshrooms for easy food. Players will be able to add signs to the coloured posts to indicate which is the best road to take to get to their builds. With the evenly-spaced main roads, I think it will be less work in the long-run getting new people connected to the network . And because not everyone can use co-ords on the debug screen easily, there will be regular navigation blocks (a sign on coloured wool with a torch on top), saying the colour and direction of the road and pointing the way to the Nether Hub. Main roads will be bigger than is necessary for riding a horse, so it will be bigger than horseable sideroads - that way it will be very visually obvious when you've hit a main road, and it'll be easier to find your way home again.