Dragon, leader of the Revolutionary Army vs who I think will be the strongest member of the Gorosei would be most fitting and satisfying to me. Back at God Valley, Dragon wasn't powerful enough to stop what was going on and Garling was at the center of the slaughter and killing games. I think it would be a cool way to allow Dragon to face someone who's both a key figure in the Holy Knights and the Gorosei. Plus, Dragon was tasked with making sure both twins made it to safety and luckily Shanks wound up in good hands, but Dragon couldn't help Shamrock, facing their father, now with both individuals at their most powerful and influential sounds good to me.
Sabo vs Shamrock could be cool.