Crabrawler is basically a Fighting variant of Corphish. While its a possibility, I doubt its going to happen and without a Fighting Z-Crystal, odds are further reduced. Plus he has had a Fighting Pokemon the last three regions(Sinnoh: Infernape, Unova: Scraggy, Kalos: Hawlucha). Odds are he likely won't have one this time around and I base what he's going to catch on what Z-Crystals he has and what Crystals he can acquire. Considering Kiawe has Fire and Lana has Water, those two types seem to be off-limits but then Ash has Litten. If Litten fully evolves, it gains the Dark Typing, hence he could potentially get a Dark Z-Crystal for Incineroar from the Totem Pokemon just like Team Rocket did(provided Ash doesn't find a Fire Z-Crystal, in which case Litten could stay as its normal self for the rest of the series).
Unless Ash gets a Water Z-Crystal of his own, Water Pokemon are likely off his radar as well so if Ash completes his team, odds are looking good that he might finally catch a Psychic, Ghost(Haunter doesn't count), Steel or Fairy type this region. With regards to Fairy, I'd put the odds at above average due to Mina while the other type being Ghost with Acerola. Mina and Acerola have yet to be introduced so those are the two types I'd pick for Ash to catch. As for which Pokemon, that I'd have to think long and hard but I'd love Sylveon to be the Fairy type(granted that would mean Ash catching an Eevee for once) while the Ghost type could be Decidueye(odds really low) or another Pokemon like Sableye.
*Edit* I should add though that if Popplio fully evolves, it gains Fairy typing, hence Lana could be the one to gain the Fairy Z-Crystal(food for thought). I also forgot to add that Lillie could be the one to gain the Fairy Z-Crystal due to Snowy gaining the typing when it evolves into Ninetales(but she would need her own Z-Ring first).