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All of the following answers are my carefully crafted yet humble opinions... jerks ;(
>New Alolan Forms
There's definitely a chance. I'd venture to say 50/50. It's an alternate reality, so it's very possible Johto Pokemon this time around get the Alola forms. New forms have been introduced mid generation before, such as Rotom's. The question is, if new forms are coming (besides legendaries), will they be released in weekly YouTube videos again? The clock certainly is ticking.
>New Characters
This has happened in every "third" version released, so I'd say 99% this happens.
>Different Storyline
For sure. The question is just how different will it be? Yellow was the same, but Yellow really is just Yellow. Nothing much to it. Crystal was a minor change with Suicune and Friends. Platinum and B/W2 were pretty big. With this trend, I'd say at least a very decent story change, especially with it being a different timeline and all, they could change characters and motives entirely. There also are a lot of loose ends from Sun/Moon, such as everything that has to do with Mohn.
>Exclusive Z-Moves for Gen 7 Starters
I don't even care about this... maybe?
>New Megas
Not happening.
>New Pokemon/New Evolutions
Doubt it.
>More forms
I could see them changing up the Ultra Beasts or Island Guardians, or even both. Maybe Necrozma interacts with all of them.
>More Customization
Definitely.
>More waifus
I bet previous waifus will be modified, but I'd say we're most likely not getting any more than 1 new good looking girl, if any... and I'm more partisan to there being no more introduced.