I don’t know about this one. If this is about banning streamers who badmouth OGS, that may be tempting but you probably don’t want people to fear losing their right to notifications if they bring up an issue. If you want to encourage streamers to use OGS, the stronger ones will have a hard time finding even games anyway. Newer streamers also need more exposure. If limits are needed, it is probably better that they are the same for all and fairly objective. What about periodic limits per streamer, maybe more generous towards newer ones? I guess in this case a few extra notifications for those who use OGS when they use it would be ok.
Speaking of limiting the number of notifications, what about randomly notifying only part of the users? This may sound like a weird idea (and probably is one), but it would effectively limit the number of notifications users get, and it would allow nuance : important notifications would go to everyone, less important ones will only go to 10% users or something. This way it may even be possible to give limited notification rights to users when they want to organize some kind of small event like a collective game review. Chat and forum already can do this job, but not all users use them so maybe this is better suited (or maybe it is worse because non forum non chat users might overwhelmingly ignore these notifications - but that might be worth a try). And next instead of notifying random users, notifications would go to those who are most likely to answer them based on past events and profile information… oh god please don’t go that way.
Another thing that I haven’t seen expressed would be the ability for users to filter notifications they get, maybe based on keywords or tags, blacklist and whitelist. Though this sounds like quite a bit of effort for dubious benefit right now but this might allow for more liberal use of notifications, since users would be able to just opt out of everything they don’t want, or opt in to what they want. Good default behavior would also be important.
The forum interaction ideas sound fantastic and don’t seem to require much work. Speaking of forum interactions (but probably requiring more work), what about notifications for polls or forum posts that are looking for player attention (feedback, polls, public lessons…)? This would have the benefit of bringing more people to the forum by bringing up it’s existence through interactive events. It would also allow people to reach a broader audience than just forum users, which I assume are a small minority of players. Overall I think more interaction between forum and main site would foster community growth. Obviously not everyone can be allowed to make site wide notifications on a whim so there should be some rules I think the idea is interesting.