The project is being developed by 3 studios and supervised by Game Freak. Its development started a while before Sun/Moon did. It's a huge project and one of the reasons NX was delayed. There is a trailer being prepared for the NX's reveal (which is really soon) but it wouldn't surprise me if they waited until Pokémon Sun/Moon get released first.

There are difficulty options: New Trainer, Adventurer, Veteran Trainer. They adjust the enemy levels, AI and the Pokédollars you get.

The Pokémon models were already ready but there are two sets of textures which are being done from the scratch. New Pokémon forms will have a HUGE role in the game.

The overworld seems to be extremely detailed and a lot of work is done so that battles will feel a lot more immersive.

Customization is back, you can also unlock items by doing achievements. There are also other achievements similar to what Pokémon GO has.

The idea is to create a character and use it on future installments. You also get an exclusive item by using the Pokémon Bank.

The game takes place in the Kanto area. Expect a complete overhaul of the towns.