I'm considering pursing a Ph.D. in CS with a focus on games.

Know any open questions or good areas of research to look into?

Lower level/ performance aware is preferred to higher level abstractions.

You might want to take a look at http://openproblems.realtimerendering.com/s2016/index.html in case you are interested in graphics programming.

There's a lot of fields within games. Is there one that interests you the most? e.g. Graphics (as above), audio, AI, animation, multi-threading, scripting languages, build systems, source control and asset management, physics, user interfaces, user input, VR, etc.

The area I'm interested in most would be close to the hardware. I'm taking OS and compilers this semester and really love playing around with llbd and getting to see the computer in action. So, whatever domains could help keep that useful..

But, from what you listed, AI, multi-threading, scripting languages, build systems, asset management seem to interest me the most. But, so far I love getting closer to the metal & am just trying to figure out what to peruse to see what interests me/ a potential research topic.

You might want to take a look at http://openproblems.realtimerendering.com/s2016/index.html in case you are interested in graphics programming.

Thanks for the link. The topics I find most interesting here are:

A modern programming language for real time graphics.
Deep Learning: A new tool for content creation and game design.
Geompetry and Material Representation in Games.