You have already mentioned it, but the custom HTML blocks would be great in allowing people to add things like different amount of columns. And page manager, yes! I love it! Support for URL rewrite would be great also, but no biggie.
Another thing that would be great is having links in editable text to be clickable (maybe with right click or a key?) That would allow for not only links to pages that are not in the navigation, but also to create multiple menus yourself. An option to have a dropdown menu of internal pages when you want to make a link would come in handy, but then again you can also type that yourself.
Sorry for spamming. I was thinking: maybe the clickable link would be easier to implement in the toolbar by simply adding a ‘go to link’ arrow behind the textfield or text whenever a link for the current selected or focused text is present.