Personally, I want a buff to Lucina's speed and/or frame data. The general idea of what I want for her is to have a real niche amongst the swordsmen.
The best niche I could think of originates from FE: Awakening. In her game, (from my experience) she is a very strong unit, and her Speed is incredibly high. I don't know how Marth fights in his game, but I do know that her technique is heavily based in her father's, (as seen in their fight in Arena Ferox, and her Shield Break animation in Smash) despite her donning Marth's name while in disguise.
(Btw, "Speed" in Fire Emblem games is not their Movement Speed, but rather how often they can avoid attacks and if they can attack multiple times when they engage an opponent.)
Now, this is all cool and all, but if you were to follow my logic and apply it to other characters, you could justify numerous and game-breaking buffs, just for the sake of being faithful to their original incarnantions. (Bayonetta and Ike come to mind) This leads me to Smash.
Lucina's closest characters, stylistically speaking, are Marth, Roy, and Ike. Marth has the tipper, Roy has the hilt, and Ike has sheer power and weight. Lucina is stuck in the middle somewhere.
This is why I suggest a buff to her speed. (like frame data) It would remain faithful to her origins and it would give her a better niche amongst the swordfighters. Of course, I don't want her to be Melee Marth levels of fast; that would be absurd. But I think frame data improvements to her aerials or tilts would be the most appropriate tweaks in her gameplay, in my opinion.
Man, I hope Sakurai reads this lol.