1. New alternative evolutions for older Pokémon.
As much as I want evolutions for Pokémon like Dunsparce or Farfetch'd, that's not gonna happen, but it does not rule out alternative evos, as demonstrated with Sylveon.
The Pokémon I think would work best for alternative evolutions are Kadabra, Haunter, Dragonair, Remoraid, Combee, Pancham, and Salandit. I would definitely enjoy a male evolution of Salandit. It might just be a physical Salazzle, but with Knock Off and hopefully Fire Lash in it's movepool, it would be a delightfully devious Pokémon to use in my journey.
2. Regional Variants for later generations.
There are still a good chunk of Pokémon out there that have potential for amazing variants. Having variants for at least the first four generations would allow for some variety, since several Kanto, Johto and Hoenn Pokémon have evolutions and pre-evolutions in later generations, like Rhydon, Sneasel, Roselia, and Chimecho. This would make up for Johto Pokémon not receiving Alolan forms in USUM, taking into account their already established nativity to Alola, and establishment as Mega Evolution users.
Water and Flying-type Pokémon are able to leave and enter Alola as they please, so they're not stuck in Alola long enough to adapt.
As for Pokémon I'd like to receive regional variants; my personal choices include Arcanine, Victreebel, Granbull, Ursaring, Donphan, Breloom, Zangoose, Seviper, Weavile, and Rhyperior.
3. Buffs to certain Abilities
Some Abilities could do with solid remedies, like Mega Launcher boosting ball, bomb, and blast moves, as well as Magician enabling the user to switch and steal items at random.
4. Ability Changes
Generation VII shows that it's possible to change already established Abilities, like replacing Gengar's Levitate with Cursed Body. I want some Pokémon that have evasion Abilities like Sand Veil and Snow Cloak as their Hidden Abilities to receive different Abilities, since evasion is banned and looked down upon competitively.
Golem, Articuno, and Donphan are awesome Pokémon, and deserve Abilities that allow them to be used in competitive formats if transferred from the Virtual Console games (R/B/Y and/or G/S/C).
Pokémon that already have Sand Veil or Snow Cloak as their main Abilities, like Garchomp, Sandslash, Glaceon, and Froslass, can just receive a new secondary Ability. It will save players some hassle with breeding, unless they want specific Egg Moves.