Notifications centre is a feature which will be released with basic functionality and new features/notifications will be added over time.
The plan for the initial release is to allow notifications to be sent by email and by Slack webhook. The notifications will also be displayed in the panel itself in an "activity stream" format.
There will be notifications for system events such as the creation/deletion/migration of websites, the outcome of scheduled tasks, organisation members being added added, etc. Adding notifications for system resource usage is dependent on a separate feature we're developing to record and graph this information so it probably won't be part of the initial "notifications centre" feature.
There will be some configuration possible to define which events are "notifiable" (by email/slack/future integrations) and which just appear in the activity stream.
It won't be possible to modify the email templates initially but this may be possible in the future.