Followers in Expansion.

Any new type of follower in the upcoming expansion?
What about current ones, will they be more customizable?
Any new abilities for them as well as new level cap?

There will not be any new Followers added in Reaper of Souls, but there will be a new Artisan called the Mystic who you can read about here.

Lyndon, Kormac, and Eirena's stories will continue into Reaper of Souls, and they'll have the opportunity to level up as well. While we don't have any current plans to add new abilities for these Followers, it's possible that we'll re-tune some of their existing abilities.