I definitely like the idea of possible Johto-Alola forms. A few come to mind quite easily...
Alolan Sunflora: Grass/Fire type, evolved to take in large amounts of sunlight and turn it into fire energy. Gets Drought as an ability.
Alolan Girafarig: Psychic/Dark type, it's "dark" tail side had to develop more due to Midnight Form Lycanroc often attacking it (needs to be able to see in both directions, etc). Gets a variant of Parental Bond (new name, same ability).
Alolan Stantler: Psychic/Flying type, it uses its psychic power (developed in response to Crabrawler) to boost its small (Alolan) wings' ability to carry it short distances through the air. Also has a bright nose. Gets Dazzling as an ability.
Alolan Delibird: Dark/Flying type, evolved due to the influence of Team Skull, this version of Delibird is not nearly as generous as its Ice-type counterpart. It uses its bag to steal, rather than sharing food. Can have Pickpocket or Moody.
Alolan Dunsparce: Poison/Dragon type, it evolved in the vicinity of Kommo-o, needing to have some kind of defence. Gets Corrosion as its ability.
Alolan Shuckle: Rock/Ghost type, after having been subject to frequent attacks by Pikipek and Trumbeak, Shuckle was forced to adapt. Can have Rough Skin or Solid Rock (yes, that's right).
Alolan Skarmory: pure Flying type, living in the same area as Salazzle, Skarmory could no longer afford to be weighed down by its steel frame. Base speed is boosted to 140 with base defense dropped to 80. Can have Gale Wings or Aerilate.
Alolan Qwilfish: Water/Steel type, evolved in response to Toxapex. Can have Iron Barbs or Steelworker.
I also like the idea of a few new mega evolutions. They could fill a few more holes in the type combination spread. For example...
Mega Yanmega, a Bug/Dragon type?
Mega Donphan, a Fighting/Ground type?
Mega Rapidash, Fire/Fairy?
Mega Zangoose, Normal/Steel?
Mega Seviper, Poison/Psychic?
Mega Electivire, Electric/Fighting?
Mega Eelektross, Electric/Poison?
Mega Accelgor, Bug/Ice?
Mega Luxray, Electric/Dark?
Mega Granbull, Fighting/Fairy?
Mega Arbok, Poison/Psychic? (think about it - the whole "snake-charming thing")
Mega Weavile, Ice/Poison?
(Also worthy of megas, but not filling holes in the type spread: Magmortar, Jynx, all Starters, Froslass, PorygonZ, Dragonite, Flygon, Bisharp, Ledian, Ariados, Escavalier, Machamp, Bewear, Mismagius, Togekiss, Breloom, Roserade, and Lapras)