Everything starts here.
The final image
Modo's internal volume was used to create the nebula. There's barely anything more to show actually. I'm using only 2 procedural noises to drive the entire appearance.
Failed attempt 1: I tried looking for some really nice forms, this was one of them but I ultimately couldn't come to grips with it. I never liked this one.
Failed attempt 2: This was another attempt at it, same color scheme, similar noises... but in the end it just never worked out. What I should mention is that probably the simpler the better.