Nihilego for Lusamine (postgame)
Naganadel for URS (postgame)
Hau starter is obviously Raichu, fuck Primarina/Decidueye/Incineroar
RR adminds have two "special" pokemon (mega or legendary), Ghetchis uses Zekrom AND Reshiram (he just killed N and stole its legendary), Lysandre uses Zygarde and its Mega-Gyarados, Cyrus uses Dialga AND Palkia (no way he would catch Giratina but catching the opposite legendary after use the first one would be possible), Maxie and Archie aren't using megas because they aren't from a mega universe but they realized the orb from the other one works for its legendary so they use Primal Groudon and Primal Kyogre. Giovanni is the only one using only one special pokemon but it's way stronger than the other ones (IVs, good strategy, level...)
There's a ceremonial-like battle at the post game (different from Pokemon League) in which you rebattle the Kahuna and they use their respective Tapu.
You can rebattle Red and Green after participate in the Battle Tree at least once, Green mentions Red can finally try that weird stones Fuji gave him so he uses Mega Venusaur/Charizard/Blastoise (the one megaevolving is the last one of the three) Green uses the Z crystals alolan Oak gave him. You can even double-battle them and they use Mega Venusaur/Charizard/Blastoise and Mega Alakazam/Gyarados/Aerodactyl and a Z move for other member of their teams
Make Ash a trainer in the game, using Alola-cap Pikachu, Ash-Greninja, Sceptile, Infernape, Dusk Lycanroc and Krookodile, he's really excited about becoming the first Alola Champion ever so he's using its strongest pokemon and you beat him and destroy his hopes again in the victory road
Guzzlord's friend is a trainer too, when you beat his team of 3 Guzzlord (or so) he reveals his name is Mohn
If you can import the 3d models from gen 6 to USUM then make all the characters with a 3d model being in the Battle Tree
You can travel to the opposite world and fight the protagonist of opposite gender
