When's the last time a new slam winner won more than a single slam in the year?  That would be Mauresmo in 2006 (though she had a final in 1999).  Serena's won multiple slams in the past few years (2009, 2010, 2012, 2015), but otherwise the last person to do so would be Henin in 2007 or Clijsters with a US Open-Australian Open 2010-2011 run (separating just by the year seems a little picky).

Throw in the Wimbledon final and a silver medal, and she's had an amazing year.  I wouldn't be surprised to see her pick up another slam next year; Serena is getting older and the other players who could really trouble her are out for reasons (I'm thinking Sharapova and Azarenka here, the other power players like Kvitova or Muguruza aren't as consistent as those two).

After the US Open, she is 1680 points ahead of Serena in the rankings and defending 1215 until the end of the year.  If Serena plays the year end championships (and I'm not fully convinced she will), a couple good results before then should keep Kerber at #1.  If Kerber makes it to the semis of the year ender, she's almost certainly captured the year end #1.

She hasn't had much success at the year end championships.  I think we'll be able to tell a lot if she does well this year; she should.  Rounding out the current WTA top 10 are: Serena, Muguruza, Radwanska, Halep, Pliskova, Venus, Suarez Navarro, Keys, and Kuznetsova.  Serena, Muguruza, Pliskova, Venus, and Keys can overpower her, but Serena is the only one who's consistent.  Radwanska and Halep play similar games, but they don't have Kerber's power and, right now, they're not playing the game better than she is.  I'm fond of Kuzy, but she's getting older and has never been the most consistent player.