We all know that Crocodile only lost to Luffy to follow the plot, as he is Low Admiral Level and that Luffy was barely at Paradise Level. However, even if we suppose that he was actually weaker than Luffy at that time, he could've been succesful on his Coup d'État, if he had allied himself with Doflamingo. In fact, they'd become the kings of the Underworld together. Just imagine two great nations such as Arabasta and Dressrosa having Warlords as their kings, one of them even having Nico Robin.
By allying himself with Doflamingo, Crocodile would get the following benefits:
Doflamingo's subordinates to help him (i.e. Bellamy).
Caesar's technology.
This, along with considering that Crocodile was actually strong enough to defeat the Straw Hats at that time, would make him one of the most powerful people in Grand Line.