In Chapter 1040 the inner monologue of Big Mom cursing Roger for starting all this was really good. She was so confident in her standing as an Emperor that when reality hit her in the face she needed a scapegoat to blame. Truly aligning with her character, but I definitely don’t think this is the last we’ve seen of Big Mom.
Big Mom wishing Roger would give her some kind of clue about the One Piece while Luffy outright refused to hear any.
And I think that’s exactly why Roger didn’t say anything. He didn’t want someone who would set out in search of One Piece knowing exactly what they’d find. He wanted someone who was strong and courageous enough to go to the ends of the earth like he did and willing to go for what all they knew could be nothing at all; because One Piece isn’t the real treasure it’s just the bait.
It could still be something of unimaginable value, but the real treasure is the lost history of the Void Century that he wanted to reveal to the world so they could correct it.
You need to be able to read the Poneglyphs to even get to Laugh Tale, so by not telling people where it was he knew whoever found it would be able to learn the History for themselves. And by not telling people what it was he knew whoever found it would be convicted enough to actually do something with the information.
He could’ve just simply revealed it, but working the world up into a frenzy over the injustices perpetrated by the Celestial Dragons wouldn’t have been enough to do anything about it. They still needed the 3 Ancient Weapons to undo what they did to even have a chance at stopping them (Which I believe to be the creation of the Red Line), and since Poseidon wasn’t born yet and Roger didn’t have very long left to live anyway he had to leave that task to the next generation.
So he used his last words to kickstart the Grand Age of Piracy in the hopes there would be a pirate out there that could succeed where he failed.
As for the reason he needed the Ancient Weapons it was to open up the world by destroying the Red Line. With the Red Line (and likely the Calm Belts created by it) the World Government has a geographical advantage by being the only people who can easily traverse between them, and by destroying Reverse Mountain, Mary Geoise and Fishman Island (with Noah to evacuate them) they can unite all the World’s oceans in One Piece.
*Theory by Unabashable