In case of a fish: What if all users should buy bread and throw it in a lake before fishing.
Fishes are growing and and you can catch a big fish (and receive money equal to a bread which fish eat).

Or this game - Fish Pond: you are paying for two fishes and you will have your link with your pond.
Then your fishes will have a babies and you will have 5 fishes overall.
Then you can invite anybody to buy 2 fishes in your pond (you will receive money for this fishes).
Then your downlines can also sell their fishes.
If you will sell all your fishes you also can buy 2 new fishes and receive 5 again.
If your downline will sell all his fishes he will bye new fishes in your pond.
If your downline's downline will buy new fishes too, your downline will have to buy some new fishes and you also will sell him more fishes.
So you will have an income.

Good game?