The types have ALWAYS been Grass/Fire/Water and they always will be.

There is no other rock-paper-scissors example that works. Yes, Fighting is offensively good against Dark, which is offensively good against Psychic, which is offensively good against Fighting again to complete the cycle.

But defensively, there's a flaw: Dark moves are not very effective against Fighting Pokemon, and Fighting moves aren't very effective against Psychic Pokemon. But Psychic Pokemon are totally ineffective against Dark types. The cycle doesn't complete. That makes them game unfair - choosing the Dark starter would make your Rival even more of a joke than (s)he already is.

And there's no other three type combination that's offensively AND defensively cyclical like Fire/Water/Grass is. So I don't see a type change happening.