I kind of agree with the events part. It just keeps dragging on for too long. I'm kind of thankful the admins are already working on avoiding this for future.
IMO the lack of variety on heroes as well as game features/modes are pretty big factors as well. Sure, we have 4 classes per 5 star potential character to choose from, but some are relatively useless compared to the stronger counterparts. Also the fact that there really are barely any 5 star potential non-valiant heroes in the game (1 for each class except healers that have two) not counting the event heroes. There's also no point using or raising the 4 star potential heroes unless you literally have nothing else to raise/use, and the 3 star heroes are all fodder.
There are also no game modes or events that highlight 4 star or 3 star potential hero usage besides maybe the Winter's Ascension event where certain trials required only 4 star or below heroes. Even then, those trials were nearly impossible too as the difficulty isn't adjusted to those conditions.
One of the biggest highlights of the game for me is being able to obtain a brand new hero I can raise and use. But it's just to easy to get them. I'm not saying that they should lower the already abysmal summoning rates but rather to increase variety by introducing even MORE heroes than what we have now. There's just not enough to keep me and likely a lot of other people busy.
As for game modes, there's really only three things to do in the game atm, and that's arena, events, and story. Events and story share the same energy resource so you can really mostly just enjoy one of these modes every single day (or week if event ladder is open and you want to get to a decent tier), and arena can be done in just a few minutes before it would take another 5 hours to do it again. There should be something inbetween that can keep a player preoccupied. Some stuff I could think of would be raids, or guild-related modes, or even another event on top of a current event that spans only a couple of days instead of weeks (and hopefully doesn't use energy...). Really, any little things that can hook players into playing would likely make a big impact in player activity.