Similar to the official Reddit app, users have requested notifications for when a post is hot or trending in a subreddit hey like. Like it does exceptionally well and is seemingly worth seeing, otherwise every decent post from 100 subreddits would be blowing up their phone every minute of the day.
I think for obvious reasons we need to constrain this to say, the front page of the subreddit. If a user posts a stupid thing that never hits the front page but barely meets the qualification, I really don't think we have to add all the computational exhaustion for looking for those kinda posts when if the user logged onto the Reddit website and scanned for it they likely wouldn't manually see it either.
So basically the "time" factor (how far back do we look?) can be just decided by checking the front page of the subreddit periodically and matching stuff to that and if it exceeds.
For what defines hotness, we could look at the top posts in the subreddit for that month, see what say, the top 10% fall in, and if a post falls into that top 10% of points it's deemed hot maybe, and send out the notification?
For restricted, private, quarantined, all those weird subreddits, ehhhh, I say we just stay away from that for now. 5 users probably care and if there's enough that care we could tackle that down the road
It also might be worth making this code more generic. Check out the Pager app (I don't feel bad about pointing at another app because I've had these ideas for awhile, it's just an easy illustration) for kinda what I mean. Not only do they support "hot" style post notifications by basically arbitrary, user-defined input (ie: you define the upvotes. 50? 100?). That might allow us (down the road) to do more cool stuff and really become a "one stop shop" for granular Reddit notifications in a way that Pager only scratched the surface with (I think it's a dead project now), with stuff like notifications if title contains a keyword, for instance.