I think you ought to try "talc" as a filler. It can help in causing polymeric molecules to move past each other ,i.e.there will be more flexibility while enhancing mechanical strength.

I love the idea of using cross linked PP fiber, but I'm not sure this ever took off.

If melt flow index is increasing then viscosity will be decrease. some fillers enhance the molecular mobility. You can try Talc as Dr Nizar mentioned.

Look at the patent literature.

Most fillers will have this performance, however you might want to check side effects like tensile or modulus properties. A compatibilizer can be added -this is usually expensive-, you even can get to the point where the price is leveled between the cheaper filler and the expensive compatibilizer. As I wrote before, look for side effects, even at long term.

I think most fillers will decrease the melt flow index. Then increase the viscosity

I think, any addition insert new, must it has been there reason , therefore, for getting on low melt flow, must addition low ratio from another polymer being choosing, condition has heat capacity and melting flow approach and little lowest Polypropylene.