[tablethnuser]

Most general advice I can give is to make sure your games have engaging core loops at three different temporal scales: moment to moment, minute to minute, and session to session.

Moment to moment is the really gamefeel stuff like how it feels good to make a line in Tetris or step on a goomba in Mario. While the player is playing your game they should constantly be experiencing these moments.

Minute to minute focuses on how engaging your game is within a session. If it's a level-based game does it have that "just one more round" feeling?

Session to session is all about having the player set something up before they stop playing, so that they look forward to the next time they can play. The most famous example is the plow-plant-harvest loop that made Zynga a billionaire. You came back to the game cuz you wanted to harvest all those crops for the rewards. But if you think about what the session to session is in Pokemon Go, the curiosity of what's in your immediate area, you can see that there's a lot of design space here.

5-7 days retention sounds like at least one of these loops is missing or broken. Don't be discouraged, getting all these things to overlap is hard.

[specialist]

Caveat: I know nothing about gamification. This is the observation of an uncle.

I agree with the commentators saying that FortNite is the newest, fastest growing social network. I spent a year watching my nephews play FortNite and Roblox. Their communication (voice & chat) with their friends is a HUGE factor. These MMORPGs are their social world.

Transposing with my own childhood, it'd be like if my friends and I had every Star Wars and GI Joe and Lego set AND awesome walkie talkies. Basically, crack for kids.

(FWIW, I spent much of the year dragging my nephews and their friends to museums, galleries, hiking, swimming, etc. They all pretended to hate it, but would nonetheless still enjoy themselves.)