Outside the box
Question for the engines, structures and certification guys:
Instead of planning future engines for x- number of fan blade separation failures and constructing appropriate static containment surrounding the fan disk -
- how would it be to support the blades themselves with an integral peripheral band or cord, providing a centripetal force and opposing the separation loading?
This solution, while increasing the inertia of the rotor could be quite a bit lighter than all the present external measures and would tackle the problem at source tending to prevent separation (and by the way to damp vibration) in the first place and easing certification requirements.
Worth a try?