Returning gen 6 veteran here, and wall of text incoming with my thoughts on the new mons and sets.
First off, the classic Dragonite+Aegislash core is easily available pre-bank. This allows for easy streaks to 50 and will probably make up the bulk of the leaderboard early this generation. I myself have already planned a team using these guys.
Second, the other way to easily get to 50, the Ultra Beasts, have been good from what I'm hearing in the discord. Most singles teams will use something like Pheromosa and Kartana with something else. These probably won't be easily sustainable streaks but they will be quick and easy to obtain.
Third, the pre-bank restrictions on Pokemon are tough. A lot of classic staples are sealed behind the bank and stuck in gen 6. For reference, no legendaries(outside of the Tapus), no Gliscor, no tutor moves, no Durant or Drapion, no Protean Greninja, and a lot of others I'm missing. For this reason, pre-bank is significantly harder to build long streaks in.
Now, my biggest point. The sets have been shuffled a bit, as with every new generation. The AI can now use megas and Z-moves, both of which can spell doom for an inexperienced player. With several exceptions, Aegislash has overwhelmingly favorable matchups against the Megas the AI can use, cementing its place as a staple of the gen 7 battle tree. There are some new sets that I can see being troublesome for certain teams.
Rotom-Heat4: HP / SpA, Modest, Firium Z, Overheat, Will-o-Wisp, Shadow Ball, Volt Switch
Z-move Overheats are nasty to deal with and Will-o-Wisp is not fun for teams without a status absorber. Lum Berry Dragons match up well against this in the lead spot, boosting once and OHKOing with Outrage.
Gyarados4: Atk / Spe, Jolly, Gyaradosite, Waterfall, Dragon Dance, Crunch, Stone Edge (Previously Gyarados3)
DD 3 attacks Mega Gyarados is no joke. With frightening attack and speed after a single boost with solid bulk thanks to Intimidate pre-mega, this set will be the death of more defensive teams. Gyarados4 will not hesitate to take advantage of your passiveness and make you pay heavily should you try to PP stall it. Additionally, the 20% chance for additional effects on Waterfall and Crunch as well as Stone Edge's increased Critical Hit rate ensure that it has plenty of chances to hax you to death.
Malamar4: Atk / SpD, Careful, Assault Vest, Psycho Cut, Superpower, Rock Slide, Night Slash
An annoying thing that will cause poorly made teams to rage. This is not a serious threat to any well-built team, but can be a nightmare for a casual player to face.
Shiinotic4: HP / SpD, Sassy, Weakness Policy, Moonblast, Grass Knot, Moonlight, Spore
Ready to roll the dice on Sleep? Yeah, me neither. If you have a status absorber this thing is child's play to deal with. If you don't, it's incredibly annoying because of its access to reliable recovery and Weakness Policy deterring use of SE moves to take it down quickly. Similar to Malamar4, most well-built teams won't have any trouble dealing with this.
Salamence4: Atk / Spe, Jolly, Salamencite, Dragon Rush, Crunch, Earthquake, Double-Edge
AoA Mega Salamence is strong. Dragon Rush flinches are annoying. I guess you can PP stall this thing if you have Gliscor with a Sub up already or maybe let it kill itself by sending out Chansey, but neither of those are appealing. The best strategy to deal with this monster is having already set up.
Now while those may not look like much, those are only the new sets. Remember that a lot of Maison sets will still show up in the Tree. (Hiya Lax Incense OHKO Walrein and BrightPowder Double Team Zapdos!) That's all I've seen just at first glance too, so there may be hidden terrors. Anyway, if all goes well, then I should have my streak begin on Wednesday!