So after about 2 and a half hours of searching old and new forum posts for possible fixes I've decided to make my own post for future reference in case this happens to anyone else. After not touching Diablo III for months I managed to work up the cash to upgrade to reaper of souls. However, upon attempting to launch the game I am met with the reaper of souls transparent splash, and then the Crash Report Screen presents itself with "The application has encountered an unexpected error". I've even tried a full reinstall but it gives me the same message. I was super hyped to be able to finish my Diablo III experience but it seems that ill have to wait a little while longer. If those that wish to help need more information on my specs don't be afraid to ask.
Turns out my Discord overlay kept the game from launching.