What are the main weaknesses of and threats facing Magic Circle firms?
In terms of threats, I think you can't really look past the rise of the US firms in London and how US firms are now dominating the market share in various different practice areas such as private equity. You could talk about the perceived weaknesses behind the lockstep pay structure that many MC partnerships have and the lack of flexibility within it which has caused star performers (look up David Higgins and Adrian Maguire who left
Freshfields for Kirkland over the last few years) to move to US firms where they can be paid significantly more as the 'eat-what-you-kill' structure is much more flexible.
There's for sure many others but I reckon that's a good place to start for your research.
Hope that's helpful!