A LOT has been revealed from leaks.
There are three new starters: Tsutarja, Mijumaru, and Pokabu. The new region's name is Isshu.
There is a new girl, I'm afraid not Lyra even though that would be awesome. The new girl's name is Iris.
Ash and Pikachu are going to the new region, however no other Pokemon have been confirmed except one of Iris's (yet unnamed) and one of Jessie's (yet unnamed)
Yeah, Team Rocket will be there too.
The surprise: Brock won't be coming. This leads me to believe he may finally fulfill his Pokemon breeder dream :D
If you want to see ALL the new Gen V Pokemon click here: