Mg is the lightest structural metal but with low ductility due to the strong plastic anisotropy and fast pyramidal-basal (glissile-sessile) transitions of <c+a> dislocations. Alloying generally improves Mg ductility. Especially, Mg-3wt.%RE (RE=Y, Tb, Dy, Ho, Er) show significantly enhanced ductility compared to Mg-Al-Zn commercial alloys. To investigate possible ductilization mechanisms and the special roles of RE solutes, first-principles calculations are used to compute all relevant stacking fault (SF) energies as a function of solute type (Y, Al, Zn) and concentration in the dilute limit. Results show that the effect of dilute Y solutes on any individual SF energy can be achieved by Al or Zn at similar or moderately higher concentrations, which does not support existing concepts related to I1 SF. Moreover, Y solutes have limited influence on the energetics of the undesirable pyramidal-basal transition. Other possible concepts for the mechanism(s) of enhanced ductility are then discussed.