I'm pretty sure custom domains aren't coming back. We're on a custom domain and I'm dying to get rid of it because I think it makes it look like Better Humans is a content marketing initiative for Coach.me (domain is betterhumans.coach.me).
However, I would like if Medium moved faster to kick off pubs that were pushing high volumes of articles without a corresponding level of quality. It probably looks like I'm bickering with the Hacker Noon publisher in other parts of these comments--and that's because I am. I think the way he publishes is across the board bad: it's a waste of his reader's time, it's a waste of his own time (he has the initiative to do so much more with his life), and it drains eyeballs and reputation from other Medium articles, which then has an effect on both my own interest of trying to deliver great articles to people and of making money.
There's an addition by subtraction opportunity for Medium where they push off the low-quality high-traffic pubs and then the .medium.com domain starts to look like a signal of quality in places like HN and Reddit.