I think the next big thing will be the complete integration of WP8 into RT or vice versa. They both have strong features and the resulting combination will hopefully be a heavy hitter. I’m thinking the equivalent of android 4.0, after 2.3 and 3.2(?) became one, and better for it.
Apart from that, I think it will mostly be usability enhancements, new/more capable APIs for developers, and features that we don’t even know we want yet. (hopefully).
Skype IS Microsofts equivalent of iMessage or Hangouts. Its integrated into Windows and WP already and 8.1 will integrate the platform more. Microsoft is changing the platform of Skype to make it better as a messaging service.
Also Cortana is a feature that you missed.