I haven't played Tri Force Heroes in a while now and if the GameFAQs board is any indication, it should be pretty hard to find people these days. This was to be expected by the nature of the game and I personally can't really complain much, because I essentially got what I wanted from this. I wanted an online Four Swords game for a long time and was having a blast playing this with different people from different Zelda communities, even sometimes with random people. I've even spent more than 200 hours with this game, which is a lot for a new Zelda.
Still... I wish that they would have tried something more lasting. For the next months I will probably keep playing Hyrule Warriors: Legends and easily top those 200 hours, because this game just has lots and lots of things to do. Adventure Mode is for the most part thrown together mass missions, but if you're enjoying the gameplay, you won't mind this too much. And maybe they could have gone further with the challenges and material collections in Tri Force Heroes to keep people playing a little longer like that.
Of course it's in the nature and structure of a Zelda game to be finite, so even with more challenges and outfits it would ultimately find its end at some point. A different approach would have been needed to keep people playing, e.g. an online highscore system. Assemble teams and try to beat the levels and challenges as fast as possible and with as little damage as possible. The key to a healthy multiplayer is competition, even in team games, and I did put many thoughts into the Coliseum mode for the same reason...
But there are also probably many people out there, who haven't done all challenges and haven't gotten all the outfits yet. They just lost interest over time. Here it would have been important to keep attention on the game with regular updates, like they did with Splatoon. It was discovered that they had four more outfits planned and even another area... Where did this content go?
Well, my theory was that one of the major updates will be linked to the 30th Anniversary as a free gift, similar to the Four Swords Anniversary Edition. However, Nintendo didn't touch the subject of the Zelda Anniversary yet, where they might do so at E3 and then release another update of Tri Force Heroes afterwards as part of the celebration. But this will be very late and they should have released at least two updates in the meantime to keep the game relevant. Even minor updates count, e.g. having one or two new outfits to try. It would also be easy to add new things to the arena or new challenges to existing stages. By just adding one challenge to each stage, dedicated players like me would go through the entire game again! I'd be happy to.
It would also keep the sales coming. It has sold over a million copies worldwide, which isn't bad - it already did better than Four Swords Adventures. But some small additions now and then would keep the game in the news, keep people playing and with that generate more sales, because more people want to join the fun. So, right now the drought feels like a missed opportunity and I hope that Nintendo didn't already abandon this gem for good.