starter topics for PhD in RL

Hello,

Just started my PhD in comp sci. Previously i worked on RL and representation learning during my masters a few years ago. I have tipped my toes in a few different projects(application in medical and whatnot), but I was wondering what would be some interesting open questions to work on? ideally either core RL with easy to use environments like Atari etc.. or something in the reasoning and LLM space.

Any suggestions, hint, helps or sources with a nice summary of the current state of research would be much appreciated.