Garling well certainly find away to close the distance on Luffy.
At which point someone will have to step up take him on in his stead.
If Shanks is indeed a Figarlan, it only makes sense for it to be him. In fact, Shanks stepping in to face Garling on Luffy's behalf might be what put puts to bed any notions of Shanks possessing nefarious intent.
Besides Shanks, I could see it being any one or combination of Dragon Sabo, Garp, and Sengoku.