I would love to see M!Reflet and Lucina(Read about this in a fanfic) switching outfits, same for Chrom and F!Reflet(I saw this actually in a fanart)
Tiamo with Darkflier outfit would look good as well.
As for Fates Characters, the outfits from Cipher series(Ryouma as a Wyvern Lord and such), highly unlikely though.
Marth with his outfit from Awakening.
Too much into the latest series i guess.