Let's be honest, the concept of "rare" Pokémon is not appealing in the least. The only real bragging rights to own is a shiny variant of something, with max EVs and IVs and an ideal nature. But that's not a challenge--that's just RNG combined with a fuck ton of soul-crushing work. There needs to be something that's fun, but difficult to get through, just so you can claim to own a "rare" Pokémon.