I think the treasure was just meant to throw the world into chaos for the World Government in all honesty. I mean, Roger and his crew were the only people so far in the OP world to ever find all the ponyglyphs as well as read them and know the world's full history. They've already clarified that the Gorsei dudes are the real bad guys and they were willing to destroy Robin's whole entire island just to make sure that their past wasn't found out.
Rayleigh compares Luffy to Roger and Luffy was a good lovable guy who cared about his friends and cared about people. If Roger is meant to be a good guy and was executed by the WG, if you knew who the real vilians in the world were wouldn't you wanna make their life hard too? (lol)
Whitebeard already stated that Blackbeard wasn't the man Roger was waiting for declaring its a D. user that he expects to find the truth. I believe the treasure has something to do more with the adventure itself or the world's real villans and past if not both.
And as far as the pirate king thing, I think thats more of a Gol Roger epithet than a real title. The first man to ever sail the entire world and conquer the Grandline should have a special epithet because they did what no other man did. Well, idk, I kind of see it like I see the supernovas title for the rookies. It could have some significance but I don't think there ever was one before him.