It looks like it's back to Water for me. Nothing massively against the others, just Water has typically always been my pick. Gen 6 & 7 were exceptions (although regretted this in Gen 6).
I cant help but start guessing at the typings of the final forms. A lot of people seem to think Scorbunny will get an electric typing, but I'm not sure where that is coming from. That said, the only other one I can see if fighting, and we've had enough fire fighting starters. Grookey I'm also unsure on, but feeling maybe Grass fighting or Grass Dark. Being a lizard, Sobble make me think Grass Dragon. But others spotted his camouflage in the trailer and say that hints at water ghost, which has logic behind it.
Oh that would be sweet! And I can make an argument for that. Besides not having been done before for a starter, Scorbunny as an athlete is lightning fast, so fast he lights the ground on fire (that's due to his fire typing, but he doesn't have the electric typing until the final form so we'd only see the start of a theme at this point, like Sobble's camouflage maybe being the beginning of ghost type). You can maybe see the beginnings of Grookey being grass/rock, since he seems to be based off of rock and roll and is seen banging his stick against a rock for a drum. Personally I think either dark or ghost is more likely for Sobble than dragon since there's more logic to them than "he's a lizard" and also dragon is still a pretty powerful type, resistant to all three starter types and even electric.