Devs, you have a gem of a game on your hands, and I believe you can get a huge influx of returning (and in turn, also new players) if you simply change the game revolving ONE GODDAMN MISSION.

We, the players, are simple minded creatures, and given the chance, we will ALWAYS optimise the fun out of a game.
The problem is that FSC (firestorm citadel mission) is by far the most efficient content to run in the game at the moment, and people WILL lose their goddamn minds if they keep farming it.
Nothing is interesting after you have done it 100 times and people WILL get tired and quit.

Here is an alternative: the arcade (specifically, launching clockwork expeditions from the gates) is brilliant, fun, and very replayable. You need to either buff the arcade runs so people get a similar amount of currency in a similar amount of time as a FSC run - simply put, currency / unit of time. Even more simply put, I want to be able to dedicate 2 hours to arcade runs and I want to get rewarded a similar amount of currency as FSC.

OR (a more lazy solution) nerf FSC (and potentially all the missions) in order to bring it in line with the arcade runs. This is because missions can be spammed and they are inherently not balanced in their current state. Instantly teleporting to a more valuable depth with more resources brings us to players optimising the fun out of the game and just spamming the same thing over and over again.

Here's my simple idea that will fix everything for you, and can also get implemented fast as f*ck:
Make a system so that if a player enters an arcade run and goes all the way to the lowest depth, he gets a flat bonus of currency (crowns / orbs / crystals) as a reward at the final elevator before returning to haven.
If you want to take this even further, you can implement these checkpoints at various depths of your choosing and give smaller bonuses on the way so that people can choose to leave at earlier depths and still get rewarded a smaller amount of stuff.
Oh, and you could also steal OSRS's system of asking the players what changes should be done to the game and what content should be addded.

Yeah, I know, brilliant ideas right? No need to thank me, instead, you should hire me as game advisor and I will revive the game for you.
Thanks for coming to my Ted Talk.

~Rey