Let's put it this way... we've thus far seen Old God activity on every continent on Azeroth.
Yogg-Saron in Northrend and C'thun in Kalimdor are the obvious examples, the high elves left Lordaeron because they heard the Old Whisperings beneath it, and of course N'zoth has been putting his tentacles pretty much everywhere in Cataclysm from inside the Rift of Aln.
Pandaria is the 4th continent on Azeroth. Fifth if you include those rocks around the Maelstrom. There are 5 Old Gods. Math.
Besides, we've got three expansions to go before we hit Level 100 and only 3 Old Gods left.
My assumption has always been that Pandaria would have an Old God behind the Sha (these "Sha Corruption" theories about Garrosh have already become synonymous with "Old God Corruption" in my mind), then we'd fight N'Zoth in an Emerald Dream/Azshara-based expansion, then we'd fight "The Old God Behind the Old Gods" in the level 100 expansion and blow Azeroth the heck up.