I join Henning in questionning the method you are following to prepare for your upcoming interviews. Are you practicing cases with case partners (experts or candidates) or are you going through the cases alone?
I think that the complexity of the case does not lay only on the difficulty of the topic covered or the challenge faced, but also in the capacity to structure your toughts, to do math quickly, to make yourself understood easily, etc. and those things can only be practiced with real case partners. Those partners can also provide some challenging cases, but I want to make sure you that you are following the right approach in your prerparation first, before discussing how to increase the complexity of the cases you are doing.
I will be happy to discuss this further with you if needed.
All the best,