The question we should be asking, is Rox still alive?
If he is, we will have our own Madara to deal with.
I mean, I have no doubt he's alive, but whether he's te madara or not, I HOPE IT IS!
honestly, the existance of such character in One Piece universe is the ONLY WAY imo for us to determine if Luffy surpassed Roger or not later down the line!
Cause Roger is dead, and WB who was his closest match is dead, and Garp who also the last person to be compared with Roger is Luffy's grandpa!
Sure, some people may suggest that now that he's old, he will be muc weaker, maybe even weaker than current Yonko. But oda can find a solution to that issue by having a DF for example used on him or his own DF allows him to stay youthful in addition to other stuff. Maybe Law's fruit was used on him...etc! All kinds of possibilities can be done to ensure that.
But I feel that as of right now, the ONLY TWO PEOPLE who Oda can manage to make them Luffy's fight that will end up solidifies his name as someone on par or stronger than Roger are:
1- Rox
2- Imu-sama
By the way, the thought of people saying that Big Mom was a mere Yonko commander level back then is LAUGHABLE! She was at her PRIME. In her 30s. Same for WB! Big Mom had a bounty of 500 mil when she barely hit 12 years old if not even less, cause she looks like a 10 when she got it LOL