Take it with a grain of salt or talk directly with my source. Doesn't matter to me, I don't work for Game Freak.

>Difficulty modes: Classic and Reborn - Classic is basically "easy" mode and has similar curves to Sun/Moon, Reborn adds several new mechanics that you may be familiar with and generally increases the difficulty across the board.
Reborn mode's mechanics:
>Level cap - You cannot increase your Pokemon's levels above the level of the next gym leader.
>Field effects - Depending on where the battle is taking place, different effects can take place. Fighting in a forest or a grassy field can cause it to catch fire, powering up fire moves. Fighting on the water or in the air causes moves like Earthquake to have no effect. Fighting in a cave causes sound moves to echo and cause more damage. These are just a few examples.
>Boss battles - The villain team (I don't know their team name) will be able to use special Pokemon, similar to mega evolutions, that the player does not and will not have access to.
Now, if you play fangames, you might recognize a lot of what I'm saying. This ties in with my source:
Game Freak pulled a Sonic Team and basically hired fangame creators to assist with balancing their newest game. Specifically, they hired Ame (lead developer of Pokemon Reborn) and gave Ame creative control over a small team dedicated to adding new mechanics and balancing the ones already in place.
If you'd like more information, you can contact Ame directly on the Reborn forums or over Discord. She has not yet signed an NDA, so she is able to openly speak about her experiences so far and even has images to show of what's come of the work so far. Having said that, it also means she's been out of the loop for most content people are actually interested.

If you've got any questions you want to ask, now's probably the time to do it. Also a good opportunity to check out what the Game Freak offices look like (it's not exciting).