It's hard to imagine Blackbeard surpassing weapon-focused fighters like Roger and WB but on the other hand, he did say his power absorbs everything, including slashes... We have to see how good it does that but one slash from WB downed him even tho he moved on later without much issue... I think he might receive great power that could make him surpass those two but he would sabotage himself like Moriah did with Shadow Asgard...
Shiryu could be 2nd most dangerous man alive, after Zoro. He was already a monstrous, bloodthirsty swordsman.
And then he became invisible and whatever the Awakening means...
To give you some perspective, imagine if Shanks and Mihawk ate that devil fruit...