Except Blizzard can be criticized as it is for having Garrosh's story be an exact repeat of Grom's. They don't need to take it a step further and have the Burning Legion even be the same ones who corrupted Garrosh.
Not an exact, but WC history repeating itself is still good story. While Garrosh's plot definamtely has some Grom arch themes, it has a lot regarding the history of the Horde too.
1. Thrall took off leaving a Hellscream in charge
2. Hellscream sets focus on fighting Night Elves, loses first battle, tries again
3. Shaman turn to dark magics. Warlocks in the old Horde, Dark Shaman in the new
4. Wood/hide architecture replaced with black metal. See The Black Citadel in Hellfire
5. Hellscream brutally murders innocents. Saurfang said he'd kill Garrosh himself if he ever took them down the child killing path his father (not Warchief but first to drink) and like-minded orcs did. Thankfully Jaina saved Theramore's kids but Garrosh didn't know that.
Honestly though being over halfway through the book now I'm not so sure it's legion. Malkorok really does seem to be legit, though possibly allied with or influenced by evil. Given the long periods of time Garrosh spends either alone or with Malkorok & the Kor'kron, the insight into both character's personality, the fact that Deathwing was vulnerable to corruption dwelling deep within the ground and Malkorok has lived there too in Blackrock Mountain, Yogg-Saron's vision of King Llane's death and involvement in it, and Garrosh's use of Dark Shaman akin to Twilight Cultists I'm thinking... N'Zoth; and Malkorok's missing fingers symbolize C'Thun and Yogg and his big scar is the Cataclysm.
I like this theory. The missing fingers and the scar would especially be nice touches if Blizzard or Golden intended that. The hand analogy, in particular, is exciting as there are five fingers on a hand, matching that there are five Azerothian Old Gods. However, considering what is going on in Pandaria, I would suggest that the corruption is due to Y'Shaarj or, possibly, the fifth unnamed Old God trying to break us apart by urging us to go to Y'Shaarj's graveyard. I believe that Metzen or Kosak stated that we would not hear from N'Zoth this expansion; it will probably return when Azshara makes her move.