So I'm going to go out on a bit of an adventure to find a list of Waystone nexuses and auxillary supply lines. Let's start cracking:
On the other extreme are the nexus points, where multiple Waystone-controlled leylines come together into one place to form a larger deluge of energy leading directly towards Ulthuan. Most were erected in easily-accessed areas, many at the heart of ancient Elven communities, so most of these nexuses can be found in the heart of modern human cities. The Waystone at the heart of the Jade College in Altdorf is one of these great nexuses, and it is fed not only by nearby 'regular' Waystones but also by much greater tributaries from the direction of Talabheim and Nuln, and all this energy flows 'downstream' towards Marienburg. You speculate from there it would flow to Castle L'Anguille, and then either further along the Bretonnian coast or directly towards Ulthuan. You don't have access to enough of these to draw conclusions, but the one at the heart of the Jade College seems like a massively upsized version of the Belthani menhirs, rather than being visibly Elven in any way.
Here Mathilde gives the criteria for the general area in which Waystones were erected and mentions Waystone Nexuses. The Jade College in Altdorf, which we know used to be Kor Vanaeth (the tower that gave the place its name forming the Celestial College's tallest tower). There are several Waystones in and around the place as well, such as the immaterial waystone in the middle of the Light College district and what I believe are a series of waystones/runestones in the Amber Hills. There's Talabheim, formerly Athel Maraya, and Nuln, which was a former Elven/Dwarven settlement. We know that beyond just the Elven Waystones, the Elves cooperated with the Dwarves to turn their major mountain holds into Waystones as well. Marienburg is a nexus and Mathilde clearly speculates that L'Anguille would be one as well. Mathilde then analyses the Fort Breackd[djfpowefhejf Waystone as an example. You're not getting me to look up its name.
Then for the sake of completeness:
Tor Alessi - L'Anguille
Sith Rionasc - Marienburg
Athel Maraya - Talabheim, Talabecland
Athel Numiel - Kislev City, Kislev
Athel Toralien - Neues Emskrank, Nordland
Oeragor - Da Howlaz territory, Badlands
Unlabelled Elf Outpost - Hergig, Hochland
Unlabelled Elf Outpost - Breder, Hochland
Unlabelled Elf Outpost - Wolfenburg, Ostland
Kazad Kro - Nuln, Wissenland
Kazad Thar - Wusterberg, Wissenland
Kazad Mingol - Meissen, Wissenland
Zakbar Varf - Ossino, Talabecland
Angaz Baragdum - Mirror Moors, Middenland
Unlabelled Dwarf Outpost - Blackclaw Lair, Dragonback Mountains
This is a list of former Elven and Dwarven settlements Boney gave me a while ago.
For your consideration, I'm going to list possible old settlements so I can have your take on it:
Mousillon is described as having belonged to the Elves initially, with the Ducal Palace being built over an Elven Tower. (Barony of the Damned)
Courounne's foundations are described as Elven. (Knights of the Grail)
There's an adventure hook in Parravon that says the following: "A Dwarf living in Parravon has discovered solid evidence that the city, particularly the castle, were originally carved by Dwarfs. This does not surprise the other Dwarfs in the slightest. However, further investigation suggests the Dwarfhold was sacked by the Bretonni, led by the ancestors of the Duke of Parravon. Such an attack would demand vengeance. The adventurers have to stop the Dwarfs before they can carry out their vengeance. And maybe that evidence was planted by someone with an interest in instability." It's uncertain whether Parravon used to be a Dwarf settlement, but it's possible. (Knights of the Grail)
Heideck in Averland has Dwarf ruins. (Sigmar's Heirs)
Realm of the Ice Queen says the following: "Elven colonisation of the Old World finally reaches the site of modern-day Erengrad. This outpost marks the furthest the Elves penetrate to the east and north."
Araby has Fyrus, Akhaba and the Sorceror's Islands as former colonies, and there's an unnamed abandoned Elven colony on the south somewhat close to El-Kalabad.
Tilea has Tobaro, Remas, Luccini and Sartosa as former settlements and Monte Castello as a former outpost. (Dogs of War)
This is a list that I conjured for Boney.
I'm not suggesting all of these points are nexuses, but I'm willing to bet a decent amount of them are at least auxillary power supplies to the Waystone network, supplying the larger network. And in this update, Kislev City and Mordheim are revealed to be either auxillary power supply or more likely, Waystone Nexuses.
In addition to all these settlements, the following Dwarf Holds provide an energy supply:
Karaz-A-Karak
Karak Kadrin
Barak Varr
Karak Eight Peaks
Karak Vlag
Karak Dum
Zhufbar
Karak Azul
I'm also pretty sure that Athel Loren and Laurelorn are Waystone Nexuses. I also have a theroy that some of the darker corrupted forests have corrupted waystones within them. For example, the Drakwald has the Black Pit and the Great Forest has Six Spikes, which are dread areas of dark magic that I would not be surprised to find out are actually corrupted Waystones. For example, there is this from Tome of Corruption Page 202:
"Certain portions of the Great Forest have mystical ties to the Marcher Forest. A blackened copse of ancient trees near the tiny thorp of Sydow serves as a portal of sorts into the vile forest surrounding the Marcher Fortress"
Then there is the Forest of Shadows, which contains Melkhior's Tower. Knowing him, he likely made his tower over a place of mystical energy. There is also the Blood Fane in the Forest of Shadows:
"Deep within the heart of the Forest of Shadows, somewhere in the triangle between Smallhof, Ferlangen, and Bohsenfels, there is a small clearing with a grim, horrifying history. At its centre is a single weather-worn standing stone twice the height of a man. It is so stained and discoloured that few can guess what the stone's original hue was. Now, tainted with untold aeons of blood sacrifices, a rich rusty tone colours the whole stone, indeed seeming to arise from within it. The stone itself has four sets of ancient iron manacles hammered into it, each rusted with time and flecked with dried gore.
The grass around the stone is littered with bones: Human, Elf, Dwarf, Halfling—even Beastman and Orc. The onceshining armour of bold knights lies rusting alongside the obscene garb of Chaos warriors. To the north of the stone, with all sockets facing north toward the Chaos Waste, sits a vast pile of skulls, both old and fresh, each of which is silent acknowledgement that this place is sacred to Khorne, the Blood God and Skull Lord." Heirs of Sigmar Page 74
There's also Sylvania and Castle Drakenhof specifically. Page 10 of 8th Edition Vampire Counts says this:
"The Winds of Magic blow strong in Sylvania, and the keeps of the nobility are all built over particularly ill-omened sites. Even the notoriously violent and fearless Stirland tax collectors wear amulets blessed by Priests of Morr and Sigmar, and go about their business in fifty-strong companies whenever their Elector Count compels them to seek his due."
And Night's Dark Masters page 88 says this:
"Castle Drakenhof squats on the mountainside far above the town that is its namesake, like a coiled serpent eyeing its prey. The jagged peaks behind it are mirrored by the jagged towers thrusting skyward from this gigantic, black edifice. Ravens circle the towers, perching on the gargoyles that ring the battlements. The castle was built by the Von Drak family, and some whisper that it was done with the assistance of the unliving. The spot chosen for its construction was long considered cursed. During the starfall of 1111, a gigantic chunk of Warpstone called the Jewel of Morrslieb is said to have impacted on the very spot. Some dark influence certainly spreads from the place and did so even before Vlad von Carstein made it the home of his bloodline."
Waystones are everywhere. The most important points can probably be pared down with some research however.