You should add new chars, like one you get for a very hard mission!
Or hitting level 50-100!
That would keep players working to get something they want.
Perhaps a Dinosaur, or a Sugar gilder.
Nemo bucks is an issue. It's a money grabber, and NOBODY likes that. Make it so
if you beat a game, every 5 times you get 100.
That would keep a good balance of a money grabbing game, and a
family friendly game about animal beating each other up.
Happy gaming