Online identity is such an interesting topic! I think you really have something in this: “I am interested in how those who engage in this develop their own self concepts”. One aspect that’s interesting is the people who don’t set out to develop anything at all. These individuals come to realize the importance/impact of their online identities. There’s so much tied into this: motivation, professional advancement. etc. But the stories that focus on individuals who unintentionally create (or even stumble upon) online identities that don’t quite match their day-to-day and planned experiences could be a neat topic.