Waystone gold, so we have a reason to involve the golden order/boys
And maybe House Miriel as well?
Capture a corrupted one from Beastmen i think.
Or just a broken one we can try to repair and not fuck things up if it explodes.
The Blood Fane should be a good candidate—it's in the Forest of Shadows, was one of the locations the Hedgewise maintained (so they know where it is and how to activate it), and is currently (at least, in canon at this point in time) owned by a Beastman who hass turned it into an altar to Khorne.
Anyway, of the six avenues of research I feel this should be our rough priority:
1) Waystone Gold + Foundational wheel
-Justification: those are the big mechanical elements that we don't quite know how they work or what function they provide, so lets get them out of the way early.
2) Tributaries+Networks+Leylines
-Justification: figuring out how to make tributaries and knowing where they are needed are complementary, so they should be done side by side.
3) the Rune
-Justification: We know it's important, we know what it looks like, and we probably know how to carve it into the stone. The only things we don't know are purely theoretical—what it specifically does, and how it does that. Easy candidate to study last, I feel.
Of course, if we run into a roadblock on any of these, we can always put it on hiatus and try something else until we have a more holistic overview, or more partners. This isn't a strict list, it can be shuffled as circumstances dictate.
Edit: Just had a thought. If Gazul discovered some of the earliest runes, and if there is a link between the runes and the script of the old ones, then the lore of Gazul might contain some information on that. If I remember correctly, Gunnars has given us an open invitation to learn the lore of Gazul so long as we swear the correct secret keeping oaths. It would be tricky trying to share that knowledge with the waystone project, but it could help with studying and understanding the rune.