MIMO is a concept that appears in many different fields, but I guess that that you are interested in wireless communications?
Coordinated multipoint and network MIMO were two buzzword in the development of LTE-Advanced. There are tons of articles on that. Some new topics for 5G systems are the following.
Massive MIMO, here's a recent overview article: http://arxiv.org/abs/1304.6690
The combination of massive MIMO and many small cells is another area of current interest:
In radar, MIMO radars and signal processing of these systems are in advance. Also, Channel estimation of MIMO systems is another one. In cooperative cellular or non-cellular sytems, MIMO relays are in progress. I suggest you to do the research about power control and beamforming in MIMO wireless communications.