Challenges of Offline Rendering What are some of the challenges of Offline Rendering? I know one problem that is faced is performance and rendering times, are there any more issues? 

Path Traced photos seem to have reached the point where it is hard to distinguish a CGI image in comparison to a real photo so being able to render a photo-realistic image doesn't really seem like a challenge anymore.

Is offline rendering just about refining current techniques to find an improvement in render times now and not so much about photo realism? 



(Grand Moff Tarkin in Star Wars: Rogue One was entirely created through CGI since the actual actor had died)
Since you've used the "path tracing" tag, I'm assuming you're thinking mainly about light transport. At SIGGRAPH 2015, there was a course on the topic of path tracing in movie production that may be interesting:
https://sites.google.com/site/pathtracingrevolution/
Some of the presenters point out unsolved challenges towards the end of their presentation, for example see the second to last slides in the presentations from Alexander Keller and Johannes Hanika.

Then there are challenges in rendering that are not necessarily tied to path tracing but also present in game engines. Triangle meshes as an approximation of smooth solid surfaces are one, the use of shading normals or even worse, bumped normals have been pointed out as a source of artifacts a long time ago: "It's not really a rendering bug, you see..."