Oh god Lucina with the recolors of Eirika/Anna/Lyn/Cordelia/Tharja
It NEEDS to happen!
I think some great costumes for her are the following:
The Yukata would really set her apart from Marth and the other swordspeople, besides Lucina actually wears one in Awakening DLC.
A palette or costume based on her great aunt
There's a lot of costume potential for Lucina:
The outfits of previous myrmidons likeLucia, Mia, Marisa, or our curren myrmidon Say'ri would fit her really good.
Other costumes that I think would look good on her:
Great lord
Bride