marco from my interpretation of the manga, is regarded very highly in terms of strength. and he is the number 1 now of the whitebeard pirates. they are MIA now, but i wouldn't be surprised if they come up again down the road as luffy's allies. after all they fought together during the war, and the whitebeard pirates are probably the strongest crew luffy could squad up with against yonkou.
i do agree the mugiwaras have a long way to go, and plenty of time to grow. luffy training with rayleigh may be to surpass him, or simply to give him the blueprint to take on the new world and get stronger with each passing adventure. time will tell. either way, luffy stands to be the next gol d roger, as he carries the will of D, which imo is key.
lets not forget, the will of D is a very ambiguous concept at this point in the anime. its been referred to numerous times by old heads on the show including nico robin, but its never been explained. one thing is for certain, people take notice to those with the middle initial D. and we have no idea about whats good with dragon or how strong dude is. i can only imagine dragon's power level is ridiculous if the gov't put out the biggest undisclosed hit out him and labeled him as world enemy number 1.
...i feel like this dressrosa arc will last thru the winter