It might be a question that has to be asked after the question “What are we trying to build?” has been answered. But I think that the questions are interconnected:
How should the site evolve?
I’ve seen that people wanted the site to be “complete”, but am not sure what that means. StackExchange basically started with StackOverflow - “A site about computer stuff”. Over time, the community grew so large and the content became so broad that now there are dozens of more specialized sites (Software Engineering, Code Review, Graphics, Machine Learning, … and of course, all the non-computer stuff).
This should not be considered to be any sort of “proposal”, but only as a question: Is the goal to start with 170 specific, small sites that resemble the SE network sites, or are there plans to initially have sites with broader topics, and then have a process of fragmenting and specializing these when the need arises?
Although I’m not an expert in “community building”, I think that having an answer here might already be crucial to “bootstrapping” a new community: When the site starts with 170 sub-sites, it could be difficult to achieve the critical mass in any of them. To some extent, this problem can be alleviated by “seeding” the sites with the most prominent Q/As from their SE counterparts, as discussed in other threads. But this will not replace a community of people who are willing to actively contribute…
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(If somebody thinks that this should be discussed separately, this post could be moved to another, more appropriate thread - similar to migrating a question from a “broader” site to a more specialized one 
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