Well Luffy did declare that he'd create a crew stronger than Shanks. Also, Shanks is a Yonkou who also wants to become Pirate King. He's not some doormat that's just waiting for Luffy.
"Yes Luffy, you've come… walk on me and you'll get through to your path of becoming King"
Luffy will have to fight for it!
Why would you want Shanks to just bow down for Luffy?
Shanks has no ambition of becoming pirate king or getting one piece of wich he already knows what it is, just like whitebeard who had no ambition of becoming pirate king.
He is not a doormat but has other goals in life then the simpleminded dream luffy has, his dream has to do with something beond raftel something his captain didn't have the time to do.
For luffy to fight someone who he is heavily indebted to he must undergo a character or personality change first, it doesn't fit the bill.His personality is consistent throughout the entire series and this would be a mayor change in character, thats why i said it wouldn't make sense.useless and stupid for two people who don't share the same dream and are not in each others way and of wich one has a huge debt to the other to have a fight just because luffy needs to be tested, well the new world is the only test luffy needs haha.