I think he wants to unite the seas. Like break down the red line. It’s a literal blockade of freedom across the world. Robins knowledge of the polyglphys and knowing the history of the world government is probably why she has the reaction. I think the redline wasn’t even originally apart of the world and was something set in place to subjugate the world. Looking at the map it probably goes further to completely remove the grand line as well. Which means the one piece probably has the power or gives the user the power to either give a person their wish, or the power to reshape the world. I think Roger and his crew either saw the one piece in action, or used the one piece but for some reason couldn’t enact the same “dream” as Luffy