I think the types of the new starters will be...
...A TOPIC OF EXTREME ANNOYANCE TO EVERYBODY IN A COUPLE OF DAYS BECAUSE OF ALL THE PEOPLE WHO MAKE POSTS ABOUT THIS NOT REALIZING THAT THE PREVIOUS GUY WROTE EXACTLY THE SAME... thank you. Read before posting, guys.
Everything that SHOULD be said about the types HAS already been said. And it hasn't been eight hours since the announcement yet. Yeah, the Fight/Dark/Psychic triangle makes sense. Congratulations, you've figured it out. Go stand in line for the counter to the right to claim your medal.
Elaborating on what jas61292
said, the types in the game are always carefully built up as not to confuse new players. At the beginning of the game, you're introduced to the basic types. The rock-paper-scissors triangle of Fire, Water and Grass. Very intuitive. Next up is the Normal type, which has few weaknesses, few resistances and doesn't hit super-effectively. In ninety percent of cases, Normal types will perform quite okay. Going for a Normal Pokémon or move will rarely fail you, but it will rarely excel either.
Next up come the intermediate types: Bug, Electric, Ground, Rock, Flying, Poison, Fighting. Here, weaknesses and resistances begin to play in. You have to learn the type chart sooner or later. For the most part, things make sense as to what is weak to what.
When you've ventured a bit further into the region, the advanced types will begin to show up. Psychic, with its powerful special attackers. Dark, with raw physical power. Steel, which seems to resist everything. Ice, residing in harsh climates, wielding powerful STAB attacks, and exploiting your lack of resistances to Hail. Ghost, immune to your trusty Normal attacks, and attacking with moves with sinister side effects. Last of them all is Dragon, the powerful late-game type with rare and intriguing Pokémon, sporting badass designs, resisted by nothing and resisting a lot.
The game is built up so that you're introduced to these types gradually. The basics first, then the intermediate stuff, advanced stuff in the late stages of the game. They would never begin with Fight-Psychic-Dark right off the bat. Those types would require significant introduction for new players. Never mind that their STAB attacks and off-balanced attacking stats would sweep through a significant portion of the early-game.
Grass-Fire-Water is a perfect and intuitive setup. It's not likely to go anywhere for a foreseeable amount of generations.