Ok this one is solving the notification counter! But it means you'd have to re-order the feed when I open the community so that stuff I already read in other places falls down, and stuff I haven't read pops up, even if it's older. Right?
Yes exactly! Displaying the NEW stuff first within the community and letting the ALREADY SEEN / OLD stuff fall down (but not hiding it).
Suppose community has so many new posts (made worse by being able to post to many at once) that it's feed becomes too big to sift through. The more popular the community is, the more likely it's feed will be huge. Shall we need an algo then?