I can't believe I read people here saying Stan is a jerk for betraying Nina (actually, failing to betray his country for Nina), when Nina has been betraying him all season! I think Stan will make a nice recovery next season because quite frankly the show needs strong rivals for P&E. One of the most interesting aspects of Season 1 was the jeopardy that Stan would figure out P&E. That was completely absent from this season (largely replaced by jeopardy from Larrick) and it needs to come back.
We know Nina will be back next season but I think it will be in the same way that Vasily came back this season... scenes back in Russia. Perhaps she's now also working at a dreary facility in Siberia where what she learned can be put to use, or maybe during her trial things she's learned about the illegals program will put P&E in jeopardy or result in orders that put them in jeopardy. Some people said the fact that Nina went out the front door and not bundled out the back was evidence that her arrest was a sham. Don't forget that's also how Vasily was taken out to go home to face the music.
Also, Nina got a hunk of cash from Oleg but I wouldn't assume she still has it. Assuming she really was arrested, I'm pretty sure the KGB knows how to be thorough searching her and her belongings and taking everything away from her. In fact the wad of money could seal her fate if they find it on her... because how could she have legitimately acquired it?
