this is my first answer to a question here, but I thought, I might be able to give you an idea, as I have successfully passed the first round of BCG a couple of days ago.
The interviews were very friendly and down-to-earth.
In both interviews the cases made up to 50% of the time, however do not underestimate the importance of the personal fit questions, please.
My first case was quite quantitative and was about restructuring the fee policies of an online reatil bank, where I was given three graphs about different key numbers that had to be examined.
The second case was entirely qualitative, where we discussed (more conversational) how a certain sports association in Germany could improve the number of spectators watching the tournaments in the stadiums.
Again, I can only emphasize that what they were looking for was structure and creativity, not a framework. At least that was how I experienced it.
Overall, what seemed to be much more relevant, was open-mindedness, being honest with your answers and a friendly, "client-presentable" approach.
Good luck and all the best,