Nor is it out of the picture for a Big Mom defeat... we're on 20 years and Luffy has already taken down the Shichibukai. He's gone through 3(Including the strongest, outside maybe Mihawk who is reserved for Zorro). The rest are either Allies, pretenders, or fillers.
In terms of Government officials, we're at the coat tail of them too. He's got his Admiral revenge match, but largely outside of the top brass. There aren't any higher ups there either. Unless they want to drag out new ones.
We're at the point were an Emperor is needed and Big Mom is the most likely target, since she is at the bottom of the barrel for them. Luffy and Shanks ain't meeting, Blackbeard is end game. And Kaido is after Luffy. Big Mom makes the most sense. Nor will it be another 1v1... just like most battles. Most likely it's going to be another massive battle with Luffy getting the final shot.
If he beats her, he takes her place, which would also put him in the position to challenge Kaido. Where is not-allies, but allies could come out for the fight.