None. 2016 was the final changes made for the car.
2013:
-Launch year, only Coupes and Convertibles in this model year
-CIC idrive
2014:
-Gran Coupe introduced
-Competition Package (575 HP) offered.
http://f10.m5post.com/forums/showthread.php?t=873975
-First generation NBT
2015:
-NBT updated to touch controller and larger screen.
-LCI headlight refresh
http://www.6post.com/forums/showthread.php?t=1067822
2016-17:
-Competition Package power bump (600 HP)
http://www.6post.com/forums/showthread.php?t=1264569
-Competition Edition Coupe introduced for MY2016
http://www.6post.com/forums/showthread.php?t=1173192
-NBT interface update
2018:
-Only Gran Coupe and Convertible models offered
2019:
-final model year
-Only Gran Coupe offered
Across all model years, the steering wheel has remained the same (2013 M5s had the old steering wheel). Really, the only updates thus far in this generation has been and likely remain:
1. headlights
2. idrive
3. ZCP
Things are a bit more complicated on the M5 side - There was even rumors that the base suspension was retuned between 2013 and 2014 and the timing for updates lagged behind the M6