After the great collaboration with square enix and tomb raider, in which you could find geocaches, i think GTA5 would be the perfect game to integrate geocaches. As you might know, there has been always stuff to collect in the GTA-series. I don't know how open Rockstar Rockstar would be for a collaboration when Groundspeak would ask them, but I thing it is worth a try
inb4 geocaching going mainstream, to many people will know about geocaching and so on