It seems to me that having "general" and "particular" wiki is the way we want to go in building a massive InterWiki system.
It works basically like this: You have a subject. Say, "Games." Then you have wiki dedicated to particulars. For example, you have a wiki for "Game Design," you have wiki for particular games, you have one for video game programming, and another for RPG game programming, and another for pen & paper role playing games, and so on and so forth.
The "general" wiki is the "Games" wiki. People go there when they want to say something about games that doesn't fit on any of the particular wiki.
When people on the "Games" wiki realize that someone's talking about something that is best served by a particular wiki, they refer the person there. The general games wiki holds most of the other wiki to be "near."
Case in Point:
Someone wrote on WikiFeatures that we can use wiki to collaborate on comics. An awesome idea, and totally interesting. But WikiFeatures is for feature ideas. So I was wondering, "Where does this go?" I knew that !WikiFutures had many ideas in it about things like "Visualization will arrive in wiki," and "Wiki will be used for education," and stuff like that. The idea sort of matched- no? But setting down to write Futures:CollaborativeComics, I realized: "Wait, no. He doesn't want to write about how, in the future, we can use wiki to collaborate on comics, and to discuss the timeline and pre-requisites for it. That's not what he wants to write about. Really, he wants to say: "This is a cool idea. Does anyone know of work like this? Does anyone have some insight into how we can do this? What kinds of features would support this? Is anyone else turned on to this idea?" Which is quite different.
So, a general wiki solves this problem. (In this case, I believe CommunityWiki is the perfect place to talk about this kind of thing.)
I think it's natural to have a core wiki, which is made with the intent of just collecting general notes on a subject. When a subject reaches a critical mass, SplitWiki. Make a note in the WikiNode. As people come in, help them find where they need to go, and figure out the system. It's near impossible to figure out the perfect organization originally. Let it grow organically, though it's also okay to anticipate and try things out ahead to. (It's always okay to fail, and get something wrong.)
With time, it is natural that particular wiki will be connected to more than one general (or "umbrella") wiki. That's a good thing.
Perhaps this idea should be renamed "UmbrellaWiki?," and talk about the general wiki.