So far the first CK2 DLC unlocked something, and that something was the ability to play as a certain religion.
Logically the EU4 DLC will unlock something, but it logically it won't unlock a replacement system.
Some possibilities:
It will unlock additional monarch commands. Maybe everyone can make regular colonies. Those with the DLC can also build prison colonies.
It will unlock an extended timeline. Everyone can start in 145X and play until 17XX. Those with the DLC can play until 18XX.
It will unlock additional event chains. The AI gets the events but humans only get them if they have the DLC. Humans with the DLC and without the DLC will be able to play in the same game, with some getting the events, and others not getting the events. It could also unlock additional choices for the events. Perhaps those with the DLC have an extra choice for the comet event.
It will unlock additional map modes. Everyone can play the game just fine with the regular map modes, but those with the DLC get the core map, a better trade map, etc.... (this one will probably create customer backlash).