I have no idea if this is Spoiler or not, I mean, nothing stops people from simply figuring out that there are NEW games coming out-
But, it's been a while that Ash has caught a water starter, and I know that twice now, each, he has had overly powerful Fire and Grass starters, so I actually think that having Grass or Fire starter(s) again would be boring...
However, the writers have usually given Ash the coolest looking starter, which is usually the most popular one among the fan-base. However, I can't really see a Water starter cooler than Squirtle or Totodile in the near future...
...so I'm undecided!
-Also, I read the page on Bulbapedia on Starter Pokemon, and apparently with Gen 4 there was a rumour that the starters would change to Dark, Fighting and Psychic. I would have liked that! Give Ash the Dark type!