I would expect it to be either Portugal, Dutch or Byzantimes. We just played Hittites and faced the Sumerians (also a possible). Playing the Aztecs CGOTM 32, would make it unlikely to play the Inca or Maya (IMHO). Another possibility would be one of the older tribes that now have new traits (England, Celts, etc), but I kind of think it'll be one of the new Seafaring Tribes on a island/archipelago standard size map. Probably either a Monarch or easy Emperor level game, using a couple of the other Seafaring Tribes (look out for the Berzerks) as opponents.
I'm expecting to see the speculation thread show up some time later today (or early tomorrow). We'll probably get spoonfed the preview information to increase the speculation amongst the seers.