I could create a blog with my own URL and branding with selected Steemit posts

Yep

Could those be from any user?

Yep

Will it allow for comments? I guess it could require Steemit signup to comment, but that would bring in new users

This will be a bit further down the line, but it's absolutely something we'd like to add in.

I'm glad you're seeing what we see - some intriguing possibilities :)