I would like to suggest that Discourse add the ability for people to enter their own credentials for one of the user-end push notification services. Such as PushOver (API / Application Keys) or PushBullet where users can then tell the forum to just send a notification to those services. Since it is all user-end or on the self-hosted forum, there is no cost to Discourse. I myself am not skilled enough with ruby to attempt this, but I think that it would make a lot of people very happy.
You can use Discourse webhooks for this already.
How would one do this? And would it be applicable to people only seeing threads that they were subscribed to watch?