In light of the recent rumor about the starters being Grass/Flying, Fire/Psychic, and Water, I wanted to throw in my two cents.
The most obvious starter trend has to do with starter typing:
Grass/Poison, Fire/Flying, Water Grass, Fire, Water Grass, Fire/Fighting, Water Grass/Ground, Fire/Fighting, Water/Steel Grass, Fire/Fighting, Water
For any given generation, there is exactly one starter that has the same typing as a starter in the generation that comes after it. For generations 1-4, the order of repeat typing went Water, Grass, Fire. In Generation IV, however, instead of a Water/Steel type we got a Fire/Fighting type. It's possible that the repeating typing order has reversed, meaning that Chespin will end up as a pure Grass-type. Of course, it could be that the order of the typing is not reversed but randomized, in which case Chespin could be pure Grass or Froakie could be pure Water, like the rumor states.
It is also possible, due to the fact that generation five completed trios of pure Grass, Fire/Fighting, and pure Water starters, that none of the generation six starters will share a type with any of the generation five starters, which would be a series first.
Another starter trend has to do with the animal bases of the starters. Starters tend to fall into three categories of design: mammals, birds, and "reptile". Although some of the starters that I would classify as reptiles are technically dinosaurs or amphibians, they seem relevantly similar. This pattern goes like this:
Although it's not relevant for generation six, I predict that in generation seven the grass starter will be a bird, the fire starter will be a mammal, and the water starter will be reptilian. If we look at generation two as the "standard" starter set (which is justified since all three final forms are pure types) we see that the "default" positions for grass and water are reptile and the "default" position for fire is mammal. Fire breaks the mold in generations 1 and 3 with a reptile starter and a bird starter. Water breaks the mold in generations 4 and 5 with a bird starter and a mammal starter. Perhaps Grass will break the mold last in generations 6 and 7 with a mammal starter (Chespin) and a bird starter. The reason this is important is because if Chespin is Grass/Flying then he provides the perfect segue for a Grass/Flying bird starter in generation seven.
tl;dr Grass/Flying Chespin and pure Water Froakie make sense
Any time someone thinks they've found a pattern in Pokemon (other than the starters always being Grass, Fire, and Water-types), GameFreak throws a curve ball at us. Whatever ridiculous pattern you think you've found? Don't rely too heavily on it.
Also, amphibians are not reptiles - though some scientists consider birds to be. --- Creator of Jay's Journey (see quote!) "It's not ten years old! Therefore, it sucks!" - Nostalgia whores everywhere
^For the game's target audience, though, frogs and dinosaurs look more like reptiles than birds do, even though they aren't.
Still, the repeated starter-typing pattern is a pretty strong one; it's true in every generation so far. I wouldn't be surprised if it was broken this generation due to every type completing a trio in generation five.