You mentioned representation, political weight and the presence of the regions. People are often apprehensive about changing the current voting system.
Another fear that is often raised, rightly or wrongly, is that the direct link between voters and their local representative would be lost. As people like to say, they want to know whom to call when they want to complain.
From your studies and what you have heard, would it be possible, in a mixed-member proportional system, to preserve that almost physical link between voters and their MP?