As a former Forum/bulletin board moderator, what can happen, over time, is alack of engagement due to a lack of new fresh content and a lack of interaction.
Forum threads turn into "ghost towns". With super old dates on the last post which scare away any newcomers.
One possible solution that we came up with back in the day, was to add gamification.
By creating a reward or badge system for active posters and rapid interaction, the forum gained new life. Combined with notifications on threads that people could subscribe to, you will have a winning formula.
People like getting gold stars or rise to the level of Post Princess! It would be fun as well to have a leaderboard and have internal contests as well for the most active! Having standard default levels already made for people who are too busy and not too creative to implement the system as well as a full customizable one that allows the admin to create the names of the levels and how many posts, interactions are required to reach a certain level.
I used Vbulletin for years...it hasn't changed in years as well.
If you can add this to your roadmap or if you have and I missed it, then thanks in advance!