[unicodepepper]

But vendors are already providing a (somewhat) standard way of displaying that content through regular notifications. If I were to build a unified inbox like this, it would look more or less like a persistent view of the notifications panel from Android.

[mkay581]

This is exactly what I was thinking. But notifications panel doesn't work for the use-case where OP wants to just check in on content that may not have been triggered by a notification.