Concerning a Grass-type because it fits a "pattern:"
This is exactly how Sakurai wants you to think. It certainly isn't impossible that we get someone like Treecko or Sceptile, but trying to jump to an answer this early is a little foolish. There maybe a pattern, there may be evidence to suggest a pattern, but Sakurai is notorious for making it look like there's a pattern and then destroying all expectations. There are two Pokemon starters in the lineup so far, a fire and a water, so maybe there will be a grass one? Maybe. The chances do certainly look better. You can have your doubts and your reasoning, but suggesting that this deconfirms characters like Mewtwo and Jigglypuff is just outlandish. That is exactly what is happening here. You have every right to think that it may happen, but again, just one of those patterns that Sakurai is taunting us with.
Had we known about the removal of transformations prior to the direct, this would have been a whole different story. Logically, we would assume that Charizard, Ivysaur and Squirtle would all return separately with Pikachu, Jigglypuff, Lucario and Mewtwo returning. there's seven right there, and most would agree that's what would happen. Then the direct would have come, we would see Greninja, and we would have all agreed the he was taking Squirtle's place, and by extension, some would think Squirtle would be leaving as well. I find it very strange that we went from thinking there was very likely going to be 7 characters from Pokemon, down to only 5/6, being Pikachu, Jiggs, Lucario, Charizard and Greninja (+Grass-type or Mewtwo.) Sometimes i wonder about the logic some of us have.