Some miscellaneous thoughts so far - this is the first time I've ever tried to take on the post-game facility without having access to flawless mons (or even the breeding chains to get something as simple as a Seismic Toss Chansey), and unless I'm majorly missing something it seems like any kind of EVing outside of 252/252 spreads has taken a major step backwards this gen.
I've done 2 streaks of 39 and 31 with Tapu Lele, Mega Alakazam, and Gyarados; both losses would have been avoided with a MegaZam that had a 31 IV and 252 EVs in Speed, but what can you do. After my first streak, the attendant gave me some assorted items like a bottle cap, a Rare Candy, and a PP Up; not sure what prompts that.
There's definitely a good amount of carry-over from Maison sets; pretty much everything I've seen so far that seemed intriguing is actually a set 1-2 mon from the Maison that I had always breezed past in the early going with Kangaskhan or Dragonite. Speaking of Dragonite, I'm not sure if you can get a Multiscale one at this juncture but running a Tapu Fini support lead could allow it to bypass Lum Berry for something like a Z crystal or Weakness Policy, and with those two you could throw in something like Mega Metagross for a Dragon/Water/Steel core that doesn't require you to transfer anything over.
I've been too stingy with the BP to figure out what the 'scout' option does, but it does seem like the 'boss' trainers don't have a set lineup and run multiple sets of the same Pokemon. Cynthia's Garchomp Mega Evolved in my first run but didn't in my 2nd.
Another possible interesting AI behavior I just saw was against the TR Bronzong set that has only Gyro Ball as an attacking move. Gyarados had a Sub up and Bronzong switched out after it used its last Gyro Ball PP - not sure if that's a coincidence or not. Definitely smarter than when the opposing Oranguru used Instruct to allow Tapu Lele to 2HKO it with Moonblast on turn 1.
I ran into something that is going to be tough for a lot of set-up sweepers to face - Mimikyu with Curse. A lot of teams are going to be hoping that only shows up in the first 40 or so battles.
I believe Araquanid will be really solid for the Battle Tree.
The defensive build with Sleep Talk/Rest/Scald/Liquidation works really well with Water Bubble.
I think the Tree-optimized version would run something like Rest, Liquidation, Sub, and another support move like Stockpile, Double Team, or Power Split. If this thing walls something, it doesn't matter if you use Scald or Liquidation to KO it; the problem is going to be what happens when it gets that 1st KO and the next Pokemon comes in. It's going to be drawing in users of moves like Stone Edge, Rock Slide, Air Slash, and Brave Bird and if you get crit or flinched it's the same as being OHKOed.