The Product Idea boards have gotten an update to better integrate them within our Product team's idea cycle! However this update does have a few unique behaviors, if you have any questions about them check out our FAQ.
Thanks for the suggestion Tom! We'd love to hear from the rest of the community if they feel like this would be worthwhile and what the use cases might be. For the time being you can achieve the same end result via search.
The search page actually has a lot of what is being requested for here. I have made a video showing some of the filtering and sorting options when you're on the search page. You'll also see that you can filter out only discussions without replies: https://www.screencast.com/t/pxOzTIljTN
I hope this helps!
Note: We will be switching to a newer and better search platform soon, but similar functionality will exist there.