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.
As of now I am expecting Fire-Fighting, Grass-Dark, and Water-Ghost personally. Such typings match the effectiveness triangle pretty well. Fighting is super effective against dark, dark is super effective against ghost, and ghost is totally resistant to fighting type attacks. I doubt we will see Water-Dragon for Sobble's 3rd form, a partial dragon typing for a starter probably won't ever happen, as that would be overpowered since dragon is resistant to fire, water, and grass, all 3 starter types, it would break the type effectiveness triangle. Fire-electric would also break the type effectiveness triangle since electric is super effective against water.
Why grass/dark? He's a British drummer, not a death metal or emo band. Also, though it wouldn't make a ton of sense, they could make grass-ground, ice-water, and fire-electric, so that ice would be effective against ground and grass, ground against electric and fire, and fire against ice with electric against water, for near symmetry. Fighting-dark-ghost isn't perfect symmetry either, since ghost is entirely immune to fighting, not resistant. That said, if Sobble were water-ghost in the third form, I could see maybe Scorbunny being fire-normal? Don't know what you'd do with Grookey then, but you could just leave him straight grass. Some starter trios have done that before, with two starters having a second type but the third having just one.
Either way, I think Sobble has to connect back to UK culture or history somehow. With two of the trio already fitting that theme, it would be weird if there were just one that didn't.