As far as Central America is concerned, they are looking to the EU model with no checks at common borders with four countries initially, IIRC, Honduras, El Savador, Guatamela and Nicaragua, I suppose with Costa Rica and Panama to follow along with Belize, presumably.
Citizens of the four can go across the borders or fly back and forth now with just a cedula although there are ridiculous lineups at the road borders so this makes sense just like in the EU, where you cross with minimal or no checks at all at borders.
I am not sure where it says they'd throw the DR into the mix since it is nowhere near there - where did you read this?