This seems a bit backwards to me.
At least on Physics, our "hottest unanswered" are hard problems that require experts in a rather specific physics subtopic, so showing the question to non-experts from other sites won't help much. I'm pretty sure the same goes (at a lesser degree, maybe) for Sec.SE, EE, Robotics, Astro, SF&F, and probably SO. I can't say as much for other sites as I may not participate on them or I may not have seen any "hot unanswered" questions on them.
So this may end up bringing even more clueless people in; which, while not harmful, isn't necessary.
Now, I could get behind showing new, upvoted, unanswered questions in the Multicollider. I discover new sites (which I have a chance of participating on) this way; I go through the realtime section on SE.com and watch for interesting questions.