Anyways, here's my thinking regarding Ash's Team:
1) Pikachu: Staple since Day 1, he's not going anywhere
2) Rowlet: First Pokemon he caught and is glorified comedy service that won't evolve due to marketing(See Piplup for more information)
3) Lycanroc: Second Pokemon he caught that evolved into a Unique Evolution for marketing purposes.
4) Litten -> Torracat: Third Pokemon he caught that points to it evolving to Incineroar at some point due to its rivalry with Royal Mask and his Incineroar.
5) Eevee: Based on the haircut, it just points to Ash catching it as I don't think any of the girls, much less the guys, would have it look like that. Hau is a possibility but I just don't think it'll be the case. Ash finally getting an Eevee would be awesome but knowing him, he'll do an Ilima and not evolve it. Granted... if he follows the character he portrays in Red from the Gold/Silver/Crystal games, it'll evolve into Espeon or Ash's first Psychic Type and would be the counterweight to his rival Gladion's Umbreon.
As for Pokemon#6, its going to be one of three things: 1) Poipole. Ash has had Pokemon return later on in the season and this one makes sense seeing as Ash did this very thing with Goodra. What's to say he doesn't do this again with Poipole? It's not going to be Solgaleo because that would be so OP, it wouldn't be funny for Ash to own a legit Legendary that is a Steel Type for a League contest. 2) Kommo-o. Ash only has a few Dragon Types under his belt and although I suspect this Pokemon belongs to Hau, there's reason to believe Ash may have this Dragon/Fighting Type because Ash likes Fighting Type Pokemon(in fact I think he's had at least one in all but Orange Islands and Hoenn region). 3) Ash bringing back one of his old Pokemon. This is the least likely of the three options simply because Ash hasn't done that since Unova with Charizard. Furthermore, what Pokemon would he bring back? Won't be a Fire, Grass, Rock or Flying types, hence that rules out a lot of Pokemon. From what he has left, we have Water, Fighting, Poison, Ground, Normal, Ice and Dragon. Its unlikely to be Ice because we have Snowy and Water probably won't happen with Lana's Popplio. If Ash catches Eevee, that rules out Normal Types and Dragon likely won't happen due to Turtonator and Hau owning Kommo-o. So if anyone was to return, it'd be either Muk or Leavanny as I see it(and again they don't like bringing back older Pokemon in favor of promoting the current region).
TLDR: Here's who I predict Ash's complete team will be.
Pikachu
Rowlet
Lycanroc
Incineroar
Eevee/Espeon
Poipole