My personal preference would also be Yogg-Saron, since his design is my favorite of WoW's Old Gods, but again, N'zoth is the only Old God currently getting "face" time in WoW, so he's likely who they'll push if they even do an Old God. Even if they decide to go the nostalgia route, that would likely mean C'thun instead. Could even time it with the release of WoW Classic, for that whole "synergistic" thing.
Also, if it's tentacles you're into, isn't that all N'zoth is, essentially? Just a mass of barbed tentacles topped off be a weird mouth...thing with eyes?