I think just about anything could work. Here's an example of a game that has a variety of tasks that are all utterly pointless, but somehow very engrossing: https://www.decisionproblem.com/paperclips/index2.html

I love his ending, as well.

So if *that* can work, then pretty much *anything* can work--as long as it keeps us busy, without overwhelming us, so we're not aware of the passage of time between milestones.

I like it best when I'm s a little uncertain as to whether I'm playing optimally--so I'm constantly wondering if there is any way to improve my game play. Usually, that means there are choices to be made--ideally choices with trade-offs (e.g. an iron gauntlet that improves your defense substantially, but at the cost of reducing your speed--both of which are important), or choices subject to constraints (e.g. you must pick skills from a skill tree using a small amount of experience points that is nowhere near enough to purchase everything). That strategizing is a lot of fun. Later, as the game progresses, it becomes very rewarding when I can see the fruits of my efforts: when tasks that were nearly impossible become trivial. That's *so* satisfying. Even the *anticipation* of it can keep me playing: I want to reach that next level because I want to experience the benefits.

Here's a game that is absolutely chock full of good ideas, if you want to analyze it: https://live.wizardbanished.com/

There's so much to be learned there! It's super addictive because of all the things he does right. (In particular, he does the task-at-hand perfectly. But that's just the tip of the iceberg. His game is a treasure-trove of techniques.)

Also, I didn't say this before, but your user interface is gorgeous. It's really well done.