While Sargeras' Avatar does exist in azeroth, his actual spiritual manifestation and current host reside in the legion's homeworld. i do not expect him to make an appearance in the maelstrom. i can also say that this expansion's development commenced in march 2009 and is expected [estimated] to be released in october of 2010.
In addition thanks to obox for the info on gilneas. I really didnt think about that but it makes sense.