I think there are two main approaches to this—the first involves picking the targets that simply excites you the most. The second way involves working backwards and figuring out which subset of skills people in your audience are most in need of. In the latter, I’d suggest directly asking your followers or contacts and possibly creating an anonymous and low effort 1-3 question survey. I’ve done this a few times and even though I generally assume I have my finger on the pulse of what people like to learn about in digital publishing, marketing, blogging, etc I’ve learned a lot by soliciting feedback from readers.