I personally love the hashtag idea for this (but am not saying that you should not provide the ability for turning them off for people who don't want them).
However, I want to push here again to make notification options match permissions, even though it's somewhat of a separate issue. For example, don't provide a #pendingsub notification option to a moderator who does not have permission to approve new members; don't provide a #pendingmessage notification option to a moderator who does not have permission to approve messages; etc. Currently, it looks like I can check those boxes in groups where I am a moderator but have limited permissions, and not those.
(Related to this, but not a notification in the strict sense, are owner messages. I can turn those on or off in any group in which I'm a moderator, but I think those should be considered permissions issued by the owner, or by a mod with permission to grant the permissions, with notification options to go along with them - in other words, as a "low-level" mod in any group I can elect to receive or not to receive owner messages, but I think that should be my choice only if I have the corresponding permission.)
In general, I want notification options to be limited to permissions.
I would also like to see a list of what will constitute a notification. Do you have any handle yet on what will be the complete list of options? Because there have been a lot of new ones requested over a span of years. In addition to files, photos, and database uploads or edits, I would like to have a notification option for the modification of a wiki page, for example.
Messages are the sole opinion of the author, especially the fishy ones.