I don't think it will be Argonia. They'll have to release it on new hardware, which like with Oblivion will cause engine troubles most likely, and Argonia is such an incredibly complex environment, it should probably be ES7. I'm personally hoping for and expecting either Elsweyr or Valenwood. They're fairly large departures from anything in the previous games, and should be wonderful environments with unique cultures.
I don't get it... You're saying Argonia is too complex, and then you say they should do Valenwood instead. Ehh,... well.. here's gigantic dense swamp versus gigantic incredibly dense jungle.
And your next point, elsweyr and valenwood being large departures from the previous games... Isn't a giant swamp filled with all kinds of mysterious, never before seen in previous games creatures, as well as being mostly unexplored? (mystical..)
And then i'm not even talking about the hist, as well as all the argonian tribes of whom we know near to nothing...