Borrowing Ideas From Other Communities
If you can identify what has worked for one community in another sector, it will probably work for yours too.
Research the same type of communities (action, location, interest, circumstance, practice) in different sectors. Identify what discussions, events/activities, and content are most popular in these communities. Then use them for your community too.
If 'Things Not To Say To A Barista' works, perhaps 'Things Not To Say To A Chef/Developer/Designer' will also work.
If 'What's your biggest client horror story?' works in a community for architects, perhaps it will work in your community of practice too.
If 'How did you react when you were told you had cancer' works in one community of circumstance, it will probably work for others too.
Don't stop at discussions. Look at what content gets the most comments, what events/activities do they use and which seem to be successful.
By going through this activity (it should only take a day or two), you have a huge list of things you can do to increase activity in your community.
You just need to a) identify your community's type and b) research communities of a similar site (but different sector!) and identify what's working for them.