Grass - the feminine one
Fire - the cool sleek one
Water - the masculine one
Going by recent trends since gen 6, this seems to be Gamefreaks new way to design, that way there should, in theory, be a starter for everyone, and each one is very different from the other.
So far we've had -
Fire - Feminine (Delphox), Masculine (Incinaroar)
Grass - Masculine (Chesnaught), Cool (Decidueye)
Water - Cool (Greninja), Feminine (Primarina)
My guess is the cool one is always the one with the marketing push, as it will have the most appeal to it, regardless of gender.
With Fire type obviously taking inspiration from the chinese zodiac, I guess we could be seeing the snake next. It's sleek, and snakes are regarded as pretty cool, especially if it gets a poison typing too.
Grass is likely feminine, so could be a deer.
Water is bulky and masculine, so maybe a shark, or an elephant at a push.
Mark this, this is gamefreaks new MUH PATTERN.