Sounds like you're reaching a point where each "Content pack" is going to look something like :
- Some new skins. Some tied to achievements, some purchasable with diamonds. (Add reward)
- A new mob to increase the "high HP mob" challenge gate. (Add challenge)
- A new card to be the goal to make passing that gate easier. (Add gate key)
- (optional) A new skill tier to unlock to accellerate Passive Income and add new Earl skills/abilities (also a form of gate key)
The question is finding the secret math between those three things to keep players hooked in.
I'm currently popping a credit every 2 minutes or so, at 1.96 Sx/sec. And it feels, to me, like a moderately OK rate. Of course my passive income rate is not what a new player will experience, because I had those glory days of essentially pissing diamonds and have a stockpile now (currently 13111)