The next release of Sigil is shaping up nicely. There is so much going into it that the next release will be 0.6.0. Unfortunately, EPUB 3 will not be one of the features making it into 0.6.0. One major change coming will be a new BookView (BV) editor. Here is an unfished preview of what it might look like.
This is only a concept preview of the new editor. One issue that needs to be resolved is the double tool bar. I haven't decided yet if I'm going to use the one in the BV pane or the global one in the window itself.