Content creation would be great. I have a few clips saved and while I probably wont change anything it just takes one person with a mass of eyeballs on em to get new people logging in.
Game player bases also tend to go in cycles, look at steam-charts for any game, goes up & down over & over. Maybe we are on the downward spiral but it can spiral up too.
Burn out is real but I think not many are concerned about healthy sustainable gaming habits. They probably burn out on many things because thats how they interact with life. Thats how its been for me in the past and im sure ill find another game to dive into like that again.
New content for the game can help a lot but there's only so many rides to go on and so many ways to ride them. How can you make it more rewarding to complete content in different ways? Challenge runs are possible but why would you do it other than an internal drive? Maybe there are but I dont know. That is how a lot of the older stuff goes. Attempting to bridge the gap seems like an easier route with good benefits.
However, I feel that no matter how much you add it can be quickly boiled down to its mechanics and ground into meaninglessness. I would love new ways of player engagement (like a way to have more structured Costume Contests, who is participating, when they will start, where, who won, & how much influ) but I dont even know what kind or how big of an ask this is.
The best I can do day to day is try to be more active/engaged with other players. Talking, teaming, leading, etc. It feels a lot like being a DM but there is no one DM, we are all mini DMs. How to keep the game going forward with "fun" is another game unto itself. The same feeling of the connections that come from D&D seem to be with me in this game too. There are some people in the community who are really good at this (often seen leading trials, challenge runs, contests, SGs) and they can really bring in and retain players.
There is no other game that lets you create your own comic book hero. A lot of people could be down to see what thats about!