What about scaling characters to the level of the highest player in the group? That way you don't have to completely redo all the mobs/challenge level like you would if you went with a universal system where level doesn't matter.
I'd also suggest maxing the scaling at lvl 45. Eg of there is a lvl 50 in your group you won't get scaled past lvl 45.
I'd also suggest maxing the scaling at lvl 45. Eg of there is a lvl 50 in your group you won't get scaled past lvl 45.