My team just launched a product on Tuesday designed to solve this exact problem.

Email has a better threading model than Slack, but email clients lack the realtime features needed for synchronous collaboration. So we're trying to fix. The idea is to let you do long-form, async and short-form, synchronous communications using the same tool, and to do it over email so you can still talk to anyone.

Our product is Shortwave (https://www.shortwave.com/) -- we had a Show HN on Tuesday. We have a video here that walks through our team collaboration features: https://www.youtube.com/watch?v=Ex_o2d1GXf4&t=5s