Hmmm...
First, I have to agree with Navaho: If you're going to do it then shift the look towards Classic since that's what most peeps are use to.
*Dal puts on his flame retardant clothing...*
Second, I like the way Threads looks and the fact that it's not Classic. My users, on the other hand, are use to Classic. So why not create a "Classic skin" that incorporates the new icon ideas. With the template approach to Threads, and all the existing Classic templates that have been created already, it wouldn't be that difficult to pull it off. You could also add an admin option to select "UBB Threads or UBB Classic" view.
Of course all of this is easy for me to say cause I don't have to write the script

One of the things I'm wrestling with from a design perspective is the amount of horizontal room to give Threads. I've removed a menu that I use to use with my Classic forum; it was causing problems before the move to Threads and I like the way a forum looks when it has all of the horizontal space on the screen. If you start adding more columns then there is less room to display the existing ones, and even less room for someone to add a side bar to their page.
Last but not least: I think you are a genius Scream because of the way you've used basic html templates to control the layout of Threads. I like the fact that I don't have to be a script guru to change the look and feel of my forum. Whatever you do
please do not deviate from the use of templates! Dare I say it: Being an administrator of a Threads forum is actually enjoyable cause it's so easy!