Making the drivetrain lighter is definately something that Mugen should do. The MBX-5 is much heavier then the Losi. However, reliability - a very strong asset of the current car - should not be compromised.
Also, the steering system should be optimised, especially the tie rods which are clushing with the ball links at full droop.
Make the lower front pin more accessible
Make the diffs leak-free wihtout the use of any mods.
Better brakes
That's all i can think for now - I'm soon i have some more bits an peaces
Any thoughts?