Not sure. While it would be cool, fun, new, and all that stuff, Grass/Fire/Water is pretty good the way it is. Plus, it makes the game go smoothly; the first gyms are always worked to be simple types (Jhoto was kinda meh, but...). Rock, Water, Grass... They all can help or hinder the starter, and they're simple enough for the first-time-Pokemon-gamer to understand. Now, if they had to mess with the gym types, what then? Would a seven-year-old who's never played Pokemon before easily grasp the concept of Dark beats ghost, Psychic does neutral, and Fighting is screwed in the early game? No. It'd take them a while. And if they used the same style of gyms as they did previously, what's the challenge or the easiness to it? They all would do pretty much neutral damage. Grass/Fire/Water is just really simplistic and easy. Not to mention, Grass/Fire/Water starters are really, really powerful; some of the best Pokemon of their types. You can already get an Alakazam, or a Staraptor, or a lot of other good Pokemon for the other suggested types.