I’m sure you can come up with a better idea than the insane nonsense that is pokemon, just think of the delivery system of the monsters a trainer spits out and do away with the tournament concept. The creatures are just like whatever the creator thinks of in a toilet session, they may be original but they really are stupid when you think about it. You could focus on creature creation a little more that the creators of pokemon and actually make them have some sense and have a footing in reality, even settle for 10 creatures instead of pokemons 1000000 creatures (a lot of toilet breaks).
I would also think about this carefully “3d open-world rpg” as building worlds is as extensive as it sounds, I’m building interiors for a 3 story building atm, and that’s taking weeks/months - so you can imagine how many 3d modelers/level designers you’re going to need. But the “fan game” is a definite no unless you acquire the permissions from Nintendo, but don’t lose hope - just do the research.