There's one thing that, if in the game (and if Alolan Forms are in it, I don't see why these won't), all but guarantees hold items: Mega Evolution
Hehehe man if this happens, I'm tempted to use a core of Mega Venusaur + Alolan Muk, since I can see a lot of teams relying on Psychic-types to deal with Mega Venusaur! Well as long as Alolan Muk still gets Pursuit at least, depends on how they treat egg moves. Hoping they add them to their movepool.
Thinking about Flying-types... Yeah RBY eliminates a lot of the standard Flying-types, but let's see what interesting ones we got!
Mega Pidgeot: Mega Pidgeot sounds a lot better than usual here. It's got a good speed tier (outspeeding Alakzam), that lovely always hitting Hurricane, and hell, it could even use something like Hidden Power Grass to hit Rock-types hard. With so many Pokemon gone, Mega Pidgeot sounds like it stands out a lot more than usual!
Mega Charizard Y: I mean this can soooorta work, but Air Slash is actually weaker than Overheat. I guess it switches in better than Mega Pidgeot, but it's not really what I think of for a Flying-type STAB. Mega Charizard Y is for other things :P
Mega Pinsir: Now THAT's what Im talking about! Mega Pinsir has a great Flying-STAB thanks to Aerliate. It also can be a potent sweeper with Swords Dance and Quick Attack. Now I do worry about the 4x Stealth Rock weakness (same for Mega Charizard Y), but they're both strong enough that they can probably overcome it.
Dodrio: Normally this thing is severely outclassed by stronger Flying-types, but here, it's the strongest user of Brave Bird, so I'm much more interested in it than usual. It's decently fast too, outspeeding Mega Pinsir in particular (Quick Attack is gonna hurt though). Might be a decent choice when the Mega Evo slot is used up!
Scyther: This thing's usefulness is gonna depend heavily on if Eviolite is in the game, and if it works on Scyther. I'm still skeptical that it'd be worth using over Mega Pinsir or other Flying-types though, but Eviolite would make it decently bulky.
Mega Aerodactyl: OK Aerial Ace is weak even with Tough Claws, so idk if I want to count this much lol.
Articuno: Articuno does have Hurricane which will deal plenty of damage to Mega Venusaur, but there's really only one thing Articuno is good at: spamming Sheer Cold :P
Zapdos: I mean Zapdos could use Hidden Power Flying or Drill Peck if you're desperate to hit Mega Venusaur. They probably won't see that coming, but it may be a bit hard to fit in. It'll depend on the avaliable moves I guess.
Moltres: Moltres has a much better Hurricane, might work if you've already used your Mega slot. However, I feel like it has a harder time justifying that 4x Stealth Rock weakness over the megas, sorta like Articuno.
Dragonite: Dragonite will be great if it has Multiscale, but if you want a Flying-STAB, it's gonna rely on Z-moves being in the game, because Fly sucks without the Z move honestly.
Well that's an interesting cast of characters to say the least, with only Mega Charizard Y, Zapdos, and Dragonite being the usual relevant ones in other metagames! I dunno why I went on this tanget tbh, but it was fun!