I'd like to see a robust offering of communication channels (E-Mail, Slack, whatever else kids use these days) for notifications, so a team's use of the product doesn't ebb away because nobody goes check it for new content (and you end up sending inane "Hey I posted this question" E-Mails to get folks to go there).
Please don't just fall back on integrating Channel notifications with global SE notifications and call it a day! If you have to be active on SE to be able to use the product properly, you'd hugely limit the group of potential users. Even in a dev team, not everyone is going to be a SO/SE user.
Notification options should be relatively fine-grained. I know that leads to lots of complexity, but it's essential in a team environment.
1st priority:
Allow users to subscribe to all activity in a question, including comments to posts not by them (by starring?)
Allow opting out of notifications for a specific question completely
Other than that, only notify as SO does now - when someone responds to my question, comments underneath one of my posts, or @s me
Global @ing of other users (regardless if they were active in a post or not)
2nd priority:
- Notify on new questions in specific tags
- User groups, notify users from group X about questions in tags Y and Z
- other complex shenanigans