I am 25. I grew up on games like Monster Rancher, Dragon Seed, Digimon, and (obviously) Pokemon.
People love the idea of customization. People love the idea of having something unique (ego), powerful (ego), and recognizable (ego). When I was a kid I loved Monster Rancher because you could raise your monster the way you wished, battle it (offline), and scale the ranks. You could be somebody, even if you were really an offline nobody.
In Pokemon, people loved having a powerful Pokemon other than the starting 3 or 4, because they wanted to be unique. Same with Digimon, etc. GTA, for example, allows people to do things they want they want, and it's a hit!
Monster Rancher has died, but it was the most fun creature battling game ever made. I propose a similar clone of it.
Clone it. Before I become competent enough to.
People will love to mix monsters, fight them online, gain fame, power, renown. Simply put, that's it.