Yeah, thematically, it seems like Blackbeard wants to continue the repeating cycle of history and Luffy wants to do something new. It's kind of like reincarnation. Blackbeard's fine with living the same kind of lives over and over again since that makes him effectively immortal. But, Luffy wants the world to change, even though that may mean he no longer "reincarnates" in the future. One wants to USE the system they're trapped in, and other wants to break that system.
...I doubt that's a conscious decision Luffy is making, because he doesn't think that deep, usually. But, thematically or metaphorically, I think that's more or less what Oda's going for. Dreams exist to be fulfilled. But, once you fulfill a dream, it ends. That probably even extends to Devil Fruits, too, which are formed by people's dreams and desires. Eventually, there really should be some way to "satisfy the hunger" of the Devil Fruits for their dreams.
In Blackbeard's view, people will always be "hungry" for more. Their greed will get the better of them. In Luffy's view, people's "hunger" CAN be satisfied, as long as you give them enough. There CAN be enough for everyone. Or...something like that.
As for what Luffy's dream actually is...I'm in the camp that it's some form of a "Big Party". It might be a little more than that. But, probably not by much. But, it seems like being the Pirate King is just something Luffy needs to do as one step in achieving his actual goal.
So, you have to ask yourself, "What is a "king" to Luffy?"
Well, Luffy's saved quite a few kings by this point in the story. And he usually gets a feast afterward. So, to Luffy, a King is a guy who can order a feast, whenever he wants.
A "Pirate King" would have no borders and no land to control. A "Pirate King" is the most free man on the sea, to Luffy. So, a "Pirate King" is a King that can go anywhere and do anything he likes. A "Pirate King" could go anywhere, and order a feast anywhere he travels.
So...chances are that Luffy's dream is just being to order a feast ANYWHERE he wants ANYTIME he wants. And let everyone eat as much as they want. Because...Luffy sees almost everyone as his "friend" to different levels. It's just sometimes some of them need a little sense knocked into them. Luffy's pretty forgiving to even his worst enemies. As long as they stop acting like jerks.
And we even kind of saw a hint about that during the climax of the Kaido fight on Wano. Luffy literally shouted it at Kaido.
(Chapter 1149)
Personally, I'm imagining a travelling party boat, where the party and feasting never stops, and anyone can join in, if they want. I really think his dream will be something pretty simple like that. Luffy's not usually that complicated of a guy.
I feel like the biggest hurdle to Luffy's dream won't be having enough "food". It's going to be getting everyone to be willing to "sit at the same table" so to speak. Old grudges and prejudices making people unwilling to "party" with each other, and let things be peaceful. Luffy wants to invite EVERYONE to his party. And that's the part that's going to cause problems. Because different groups have problems with other groups even existing.
...That and forgiving Akainu. Because Akainu is the one guy I could see Luffy having a big personal problem with himself for what happened to Ace. But...Hibari and Koby might end up helping smooth over that relationship. I could see a scene of forgiving Akainu being one of the last big "tests" of Luffy's dream, by the end of the manga. To see if he REALLY believes that EVERYONE should be invited to his big worldwide party, no matter what happened in the past. To see if he really believes that old grudges aren't that important and that moving on to the future is more important.
But, we'll see. I could be wrong. But, I do feel like this is the most likely case.
