I would highly, strongly, enthusiastically recommend The Leftovers next. I am currently plowing through The Americans and loving it, I'm on S4E2 atm and will probably be finished by next week. It's the best show I've seen in a while, and while I wouldn't quiiite put it in the god tier realm, it's pretty damn close and my opinion could change by the time I'm done. But The Leftovers is easily in my top 5 shows ever with Sopranos, Wire, BB and GoT. And while very different from Americans, there are similar strengths in the world building, relationships, dialogue, strong episode/season themes, and I even see some parallels between the Philip/Elizabeth dynamic and Kevin/Nora. It's the only show I could think of moving on from the Americans that wouldn't be a disappointment. I don't say this a lot about tv, but The Leftovers changed my life a little bit as far as how effectively a tv drama could make me think, feel, and reevaluate my perspective on relationships and reality in general.