Before I go into my choices, I want to say that I've begun challenge myself recently in each generation by only using Pokemon that were introduced in the region, as such this team is 100% Gen 7, this team was also made with the intent of excluding Legendaries and Ultra Beasts since I want to Soft Reset for the right natures and competitively train them in the post-game
Primarina: Originally I was planing on using Rowlet as my starter, however as I saw the backlash Popplio was getting, I knew GF was planing on making it into the next Greninja so I thought "if they make an enticing Grass or Water type, that will make my decision", along came Lurantis and I immediately chose Popplio. Post Data-mine I was a bit sad that most of the new pokemon in this generation is relatively slow, Primarina being no exception with 60 Speed, but this seava isn't going to be slowed down anytime soon, as it is hard hitting and quite bulky too. A moveset of Sparkling Aria, Shadow Ball, Ice Beam, Moonblast should get me through the game.
Lycanroc: I was so into the little pupper that is Rockruff, when he first was revealed in CoroCoro I knew he was going to be on my team, A rock type relatively early? what could possibly go wrong. I was already set on getting Sun I was super glad to get the sleak Midday form instead of the weirdly proportioned Midnight form. Post Data-mine it was obvious that Lycanroc was going to be a fast physical attacker who can do a lot of powerful attacks, sadly can't say the same about his movepool, which doesn't have a lot of moves. Speaking of which, a moveset of Accelerock, Stone Edge, Crunch, Stealth Rock should get me through the main game and be a useful base for competitive battles.
Tsareena: Like many people when we first saw Bounsweet, I thought it was either going to be a Cherim clone or a one off pokemon. During this time I chose Lurantis to be a team member due to several similarities it seems to have with Lilligant, when Tsareena was anounced I was in a pickle as to who to place on my team, the cunning Lurantis? or the Kickin' Tsareena. It wasn't until Post Data-mine that I decided on Tsareena, as she had better stats, abilities, and movepool than Lurantis. I think Trop Kick, High Jump Kick, U-Turn, and Aromatherapy will do for the main playthrough.
Kommo-o: Ever since Jangmo-o was revealed I knew it was going to be Alola's psudo-legend, and lo and behold I was right. although the only thing I don't like about it is it's shiny color, Kommo-o hits hard and has relatively good speed (anywhere near 90 is good in my books). the unique typing of Dragon/Fighting is as good as it is bad. Post Data-mine and he turned out to be relatively well rounded, good defenses and offences gives him an edge over several problems he might have. Outrage/Dragon Claw, Dragon Dance, Earthquake, Sky Uppercut should work well enough for him.
Golisopod: Originally Mudsdale was going to be on this team over Salazzle, however due to typing he was dropped. which was the same reason Golisopod was put onto the team. Upon seeing him the first time I knew he was going to be a physical powerhouse, but I didn't put him on my team yet since I didn't know how well he would handle. Post Data-mine, he proved to be just that, a Physical powerhouse, although slow he seems to have the bulk to survive a hit or two before his ability kicks in. Leech Life, Liquidation, Sucker Punch/First Impression, Rock Slide may work for it.
Salazzle: As stated earlier, Mudsdale was origonaly going to be on the team, but after thinking through it and seeing Kommo-o's x4 weakness to Fairies, I knew Mudsdale had to be dropped, and among all of the pokemon that can fill in his slot, Salazzle was the only who counteracts this glaring weakness. Since I chose her after the Data-mine I knew what I was getting into...but to be honest, I thought it might drop one of her types to get a Dragon Type based on the posture of her. Anyways, Dragon Pulse, Flamethrower, Sludge Bomb, Nasty Plot should work out for her.