I cannot speak directly to the diploma vs Masters in GIS question. To me it would be a function of your job prospects. If you are currently working in GIS and can learn on the job, that is a great way to go. If its that you are looking for ways to enter the job market, then a Masters can be a good asset to have. But really that will just help open the door to opportunities. Personally, what I look for when interviewing a candidate is passion about communicating information via maps, someone who has demonstrated that passion by creating maps either online or in print.
Regarding the future of GIS, this video does a pretty good job of providing a perspective on that from Esri staff. What i'm interested in related future of GIS is to improve the ways in users interact with a map to gain information, essentially the usability and design of mapping interfaces.
Regarding the underground 3D mapping, we do currently have the capabilities to do underground mapping in 3D. I'll share an example shortly.