RE: Your hopes for 6th generation?
The thing with pre-evolutions is that it allows GameFreak to make a strong Pokémon available early on, thus not always having to wait until around the 7th gym to own a [insert strong Pokémon]. Onix however, is weak enough to be a wild Pokémon from the start, so it probably won't need a pre-evolution for that reason.
There are also Pokémon like Kecleon and Lapras, which are more a novelty Pokémon, they usually don't get pre-evolutions either.
And what is wrong with a standalone Pokémon? I think it's more pleasing to have them every now and then scattered through the Pokedex, they are becoming more rare as more evolutions are added every generation.
The thing with pre-evolutions is that it allows GameFreak to make a strong Pokémon available early on, thus not always having to wait until around the 7th gym to own a [insert strong Pokémon]. Onix however, is weak enough to be a wild Pokémon from the start, so it probably won't need a pre-evolution for that reason.
There are also Pokémon like Kecleon and Lapras, which are more a novelty Pokémon, they usually don't get pre-evolutions either.
And what is wrong with a standalone Pokémon? I think it's more pleasing to have them every now and then scattered through the Pokedex, they are becoming more rare as more evolutions are added every generation.