I don't believe add-on's will make forum more fun and engaging ....it's about what they like the most about that site - Easy features to understand and use ...contents they like to read or they are looking forward to and overall a community which actually makes them feel like family . Atleast that's what i have learned over years got to know lot of my board members and now we meet up every now and then share some drinks talk about site what we should focus on more to make people want to join .
Also remember sometimes it depends on what kind of community board you are running it all depends on niche cause every board has it's own purpose which might require certain add-on's .