Weather is back from Gen 5 with a vengeance. Definitely not as overpowering, but there's quite a few weather setters here that I'd like to address here, since quite a few Pokemon have some new tools to use.
First off, Rain. Since Pelipper and Politoed do not exist, we instead have Whiscash and Simipour. Whiscash is especially important because it functions as a Pelipper lite, albeit with tools you may not expect. Firstly, Whiscash's Special Attack was buffed, so now you do have to be wary of Hydro Pump--but what's more important is its access to Stealth Rock and Spikes. Although Whiscash doesn't have a reliable recovery option, Stealth Rock is still equally important in a meta like this, where Offense has been a lot more ramped up. An honorable mention goes to Simipour, who is a more offensive take on Whiscash with Scald, even though it has to use Hidden Power as coverage. Notable Pokemon that get advantages in the Rain are Mega Swampert, Golduck, Basculegion, and even Lumineon.
Second, Sun, and it has been tremendously buffed. The obvious automatic setters are Ninetales and Simisear, with Simisear having better offenses and Ninetales being better in the Special Defense department. But the big one I want to talk about is, surprisingly, Cherrim. Not only has Cherrim's stats been buffed, but Flower Gift now automatically sets Sun alongside its previous effects. Not to mention Cherrim's stat buffs, with its Sunshine form being especially prevalent with 80/105/60/67/105/105 stats. Good luck surviving Petal Blizzard due to the 1.5x buff Cherrim gets. It is lacking in the coverage department, however, being completely unable to hit Steel-types, but do not forget that it has support. Mega Charizard Y is a great example, hitting the Steel-types Cherrim is unable to with its Fire moves, not to mention automatically setting up Sun for its team. The only reason why Megazard Y wasn't shown off is because I couldn't find an animated sprite for it in the Gen 5 style. Notable Pokemon that get advantages in the Sun are Hisuian Lilligant and Venusaur.
Third is Snow. Do you want an automatic Snow setter? Sorry, but your only option is Alolan Ninetales, but even then, it is surprisingly good at its job. Fast Aurora Veil is no joke, and Alolan Ninetales itself also has some decent utility moves. Mega Abomasnow is Snow's resident sweeper, as it and its base form now have Slush Rush instead of Snow Warning. Don't get me wrong, though, there are definitely some bulky options in the Snow, Walrein is a good example of such, and even Avalugg and its Hisuian form can take advantage of it with Ice Body. I'll leave it off here, since Snow is easily the least influencing weather in this mod, though I definitely think someone could build around it if they had the time.
Finally, Sand. There's two Pokemon I want to mention here. First, Tyranitar and its Mega. It's pretty much THE standard for Sand, as it's always been for quite some time now. Pursuit has been the defining factor of TTar ever since it was introduced, and let's not forget how bulky it can get when it Mega Evolves, and it doesn't need to be the only Mega due to the ability to bring multiple Mega Evolutions per team--Mega Garchomp has Sand Rush now, and it can do quite a lot of damage if you're not prepared to handle it. Hippowdon is also a bulkier Sand setter with reliable recovery in Slack Off if you want to use it, but the real highlight is Bastiodon, who has gained Sand Stream. Bastiodon is stupidly bulky, and now that it has Sand Stream, it is even more bulkier than ever before, and the Steel type can be invaluable for moves that Tyranitar would otherwise be weak to. There's not too many Sand users besides Mega Garchomp, but other Ground, Rock and Steel types definitely come to mind.