So popular social media communities 4Chan and Reddit got their hands on a Citra copy of the game and have played through the game, revealing most of it's content.

Instead of being proper sequels like Black and White 2, Ultra Sun and Ultra Moon are just enhanced versions of the original game. There have been some minor tweaks to the story, notably an entire new sub-plot involving Necrozma and a new end-game sub-plot involving Rainbow Rocket but otherwise it's the same story.

They also revealed the new Ultra Beasts.
Poiple, Nagandel
Stakataka
Blacephaleon
Zeraora
Ultra Necrozma

There's also been a few changes to the Island Trials and the Elite 4, but the majority of the gameplay is still the same. You still run into the same problems where even members of the Elite 4 don't use six Pokemon.

I've been buying Pokemon games on launch since Pokemon Black, these maybe the first set of games I skip out on. Ever since these games were announced, I was hesitant to buy them and was hoping they would try and improve upon the originals. Instead this is just a lazy GotY edition, that unlike previous games, doesn't address the flaws of the original Sun/Moon.

I’m not looking forward to this much, but will still get it at launch since I get every Pokemon game. Wish it was a sequel instead of just an enhanced remake basically.

I had assumed it was just a Yellow Platinum style game rather than a real sequel. Shame I wasn't wrong.

In fairness, you had every right to think that way going by the promotional material. I don't know, I just thought since they went to the trouble of releasing two and there seemed to be so much content left over from the games that we would see more changes.

Since everyone seems obsessed with alternative universes (thanks Rick and Morty), I thought they would explain away the Lillie sub-plot and focus more on Guzma and some secret plot involving Necrozma. A lot of the main game would remain the same, but everything would be refined and there would be minor alterations here and there. I also expected to see a bunch of features return but at most we got the O-Powers back and that was that.

In a way, I'm glad Nintendo have put in such a minimal effort for this game. I still have a copy of Pokemon Sun that I've barely touched and I didn't put anywhere near the amount of hours into Moon like I did Generation V and VI. At least I don't feel like I'm missing out and can play through my backlog.