This post to suggest a modification in pet system.
To introduce, i think there is one problem in pet system : it's totally impossible to get a real pet without cash (actually) and so to really play with the pet system (formation etc...). The main problem is that better pet (with more stars) are really to much better than pet with less stars. I agree with that more stars on pet give more better propreties, but better propreties at the same lvl + more lvl enabled make a real horefull difference between people who can spend many money or not. Even with time and patience, a player who can't acquire a pet with many stars can't really play with that system and i think it's bad for the game.
I suggest one of these 2 possibilities (only one of these 2, NOT both) :
1 - Pet with more stars keep their better properties, but all pets can upgrade 80 (the difference in stats would stay enormous but everybody could have a decent pet in that way).
2 - If someone reach all lvl max of its pets, the next pet (the more little he haven't) will be automatically unlock. For esemple, people who have only Kit Master, if he upgrade lvl25 Kit Master he unlock Slinging Fox without do nothing more than clicking on "Unlock" ). I think that's the better possibilitie, in that way, with patience and a really long long farming all people could get all pet and that farming will create a real aim to upgrade all pet and not only focus one. It could be really enjoying i think.
I think if you choose one of these two possibilities, more non-vip players will stay more in the game, and if there is more non-vip player, vip players stay too.