I'm fairly certain that none of the addons will be released in the next version. I think it will be all about the performance and memory issues mainly. But who knows? I can only go on Lee's blogs and any news on the forums and such.
Not seen too much on the importer and tbh am not really that bothered about it myself. It will make things easier, but it's not really a life saver for me.
From what I have seen of the Constructor it is not ready to release as yet (although, even in it's current state it would still be fun to try). So that is doubtful. As far as I know, little work has been done on it since the last video, as the coder was moved onto converting the grass code into C++. This is only going from forum posts and Lee's blog.
From all reports I imagine 1.6 will be faster, have more efficient memory use, less buggy and have proper support for the game store. Anything else, they have kept under their hats