I'm going by gameplay merits. Having Lucina as a slot is just as dumb as having Male Wii Fit Trainer as a slot gameplay wise.
as you seem to be somebody who sees no potential in anybody who doesn't drench uniqueness, I can see why you feel that way.
That logic only works with alternate versions of existing characters. Lucina isn't an alternate versions of Marth. She isn't a Toon Marth, she isn't a female Marth.
So NOTHING about her resembled Marth? Not her clothes, not her color scheme, not her sword, not her body? I suppose next you're gonna tell me you can't see the similarities between Mario & Luigi.
Does she share similarities to the Marth that was in Fire Emblem Awakening? Yes, because that was the entire idea in Awakening. Does she share many similarities to Marth in Smash Bros? I dunno, you tell me.
I see a primarily blue/yellow color-scheme, a Falchion (even then, it's design changed), and a cape. Visually, those are the only similarities I really see.
Easy. Make an overly optimistic claim that "he could be different if Sakurai really tried."
Ho ho, didn't need the sarcasm there pal. But hey, he really could if he wanted to. I mean that is if Luigi and Rosalina.
We're not just looking for ways to make similar characters co-exist. We're looking for PLAUSIBLE ways to make them co-exist. Like Pikachu and Pichu co-existing without being clones would be an example of something not plausible. A character who has more similarities than differences will end up being a clone according to every character in Smash that fits that description. I've yet to see a character with differences in less number than similarities that has not become a clone of some sort. And Lucina of ALL people being that exception is absurd.
The real exception is Luigi. Lucina could be an exception, I mean isn't Rosalina an exception too? There's no reason why we can't get Lucina if the only reason preventing her is is her similarities to Marth. Which Chrom has similarities too, baring the visual one. Nothing is stoping Sakurai from making Chrom a Marth clone, right? But he could if he wanted to.
Shouldn't be writing off characters because of potential similarities with other characters, like we did with Rosalina.