"What lens should I get next?" is a common question I am asked and it is not a simple answer. But one little trick i use to help me decide which lens i should buy next is to examine my current lens/focal length usage. By looking at this information you can see if you need a direct lens upgrade, or to go longer/wider... I hope this video helps! :)
This is a video about how i choose what my next lens might be... I use lightroom to analyse my usage via this cool sight: http://www.lightroomdashboard.com