I've seen reasons against this, saying that grass/fire/water is the choice starter because it's balanced or whatever, helps new players adjust. but other reasons are because the bright colors help new young players identify more, and help newer players identify the basis of typings more easily. So how about in this game, maybe after you beat the game, you can travel to a smaller sub-region, where you get a new starter of your choice between psychic/fighting/dark trio, and a long with the sub region, are, past gen pokes, and some of this games pokemon on the island you couldn't get before post-game. You cannot bring your old pokemon to the island until you beat it, so it's like replaying the game without starting a new file!!!
Everyday the sun comes up around her, she can make the birds, sing harmony! http://i.imgur.com/VBsWOu2.png <3 Ain't no woman like the one i got..
I can see the one argument in favor of starters having Fire/Water/Grass as their primary types - it's intuitive to new players - but just that one.
Secondary types can be nearly anything that doesn't completely wreck the triangle (in theory, what's to stop them from wrecking the triangle? But I don't think they'll do it. Empoleon becoming a Steel-type does not wreck the triangle - Torterra's a Ground-type, after all.) The fourth gen's two-way triangle could work even better with Fighting/Dark/Psychic dualtypes - let's just keep the Fighting type away from the Fire type this time around.
I still find the "unbalanced" argument ("Dark is immune to Psychic instead of just resistant! How unbalanced!!!1") to be ridiculous - the starters aren't really meant to be balanced (if you disagree, ask Venusaur how it feels about Charizard's double-resistance to Grass), made all the sillier by the fact that most of us are talking about final form two-way triangle starters (OMG how can a Fire/Psychic Pokémon ever hit a Grass/Dark Pokémon! Dark is immune to Psychic! And Grass is immune to Fi... oh wait no it isn't.)
Anyway, as I keep pointing out though no one ever seems to pay attention, the Rotation/Triple battle trainers in BW1's Opelucid City use a Simisage/Simisear/Simipour (Grass, Fire, Water), and a Mienfoo/Pawniard/Elgyem (Fighting, Dark(/Steel), Psychic). Clearly GameFreak recognizes the triangle.
Creator of Jay's Journey (see quote!) "It's not ten years old! Therefore, it sucks!" - Nostalgia whores everywhere