I think that piece of concept art that leaked Decidueye, Incineroar and Primarina could hint to what direction Ash's Alola team may go, especially since Ash and Mallow were seen in one of the drawings on Incineroar's section of the artwork.
The drawing seems to suggest Rowlet will remain unevolved while Litten goes all the way to Incineroar. Unfortunately I have a problem with Litten fully evolving because Ash has already had two Fire-type starters as his regional ace (Charizard and Infernape) and adding a third one would just be overkill for me and unfair to those who wanted Rowlet to be the Alola ace.
Rockruff wasn't shown in that artwork, but I could definitely see it evolving. Out of which Lycanroc form I could see Rockruff becoming, as much as I love Midnight's design, Midday is probably the more fitting one for Ash.
As for if Ash should catch a Water-type in Alola, I am really feeling that it's unlikely. While he did become friends with a Wimpod in one episode, he didn't catch it, and most of the Water-types of the new Pokemon lineup are either unfitting for Ash (Pyukumuku) or were given to another main cast member instead (Popplio, Mareanie). I feel Dewpider is probably going to be the only suitable contender here, since I like its design and I'd also love to see Ash get a Bug-type once again.
