It must have to be something you only can achieve after becoming pirate king so my guess would be is that he needs to unite most pirate crews of the world to achieve the rest of his dream. From the crew´s reactions it has to be something that seems almost impossible but also something very desirable (Chopper praised him and Franky also sounded very enthusiastic about it).
The "party all over the world" theory a lot of people believe in is a bit too simple in my opinion, it may be the final part of his dream but Usopp´s reaction would be way less concerend if that would be all about it.
Ace wanted to live as free as someone can live in this world, so this may have inspired Luffy.
I think his dream is to make every people in the world live as free as they possibly can. And because of that Luffy´s ultimate goal is to unite the world! Probably by blowing a hole into the Redline! So that all the oceans can be connected and all people can move freely all over the world. This new world would be called One Piece!
Diving deeper into my theory this would also mean to guarantee real freedom for everyone the world government has to fall.
As for the place where the hole in the Redline could be made and where it might possibly already existed back in the void century my guess would be Rivers Mountain as that whole mountain looks like it was built artificially. The initial Redline might actually have been two seperate islands like New Zealand, with a north island, a south island and a connection between all four oceans (blues) inbetween.
Luffy would of course not know about that last parts of my theory but the dream to connect the world itself seems simple enough for a child to come up with. Roger could not fulfill Joyboy´s dream (which was the same as Luffy´s) because he didn´t have enough time left and did not make enough connections with other pirate crews to make them follow him.