It's a pretty natural course, honestly. The game's been around for over a decade and still has active players, that's pretty fucking well done. But, the problem is that besides events and cosmetics, not much new is happening. I can confirm personally that even though the core gameplay of Toribash is very interesting and well executed for what it is, it gets dull after so many years.
The way you control your tori is simultaneously a strength and a weakness here, because while it's almost entirely unique, the complexity makes it really difficult for new players to pick up, and even more difficult to make sensible adjustments to. You can make additions, such as environmental objects, which absolutely were a huge factor in keeping the game alive, but making significant changes to the player model is exceedingly difficult and often ends up being tough to control effectively even for veteran players.
So, how to revive it? Well, i'm thinking Nabi and the staff members already are doing or have done what's doable (and i can confirm ES is hard at work making events all the time). TB Next would be my best bet, but that's kind of out of our hands. That besides, iunno, invite your friends and beat them up?