It could be tons of topics depending on your personal interests, just to name some that I find interesting (numerical modeling related topics):

-Dynamic stability and damage of tunnels under strong earthquake motions

-Application of SRF method to assess the static stability of tunnels

-Comparison of limit equilibrium and Distinct element methods' performance to assess wedge instabilities.

-Blasting that could impact or reactivate nearby joints

-Performance of installed rockbolts under certain types of blasting

-Time-dependent effects on squeezing ground

-Statistical study of face tunnel stability

If you relate the above-mentioned ideas with mining engineering itself, you can end up having very specialized topics, with applications to block caving and open stope mining.

I hope you can find some inspiration in these vague ideas. Best regards,