While I'd prefer for Lucina to become "EX Marth," I'd probably take the same approach that a lot of fighting games take while differentiating shotoclones - especially since games like Icons and Rivals of Aether are kinda turning "tipper swordfighter" into the shotoclone of the platform fighter genre.
So how do traditional fighters differentiate shotoclones? By emphasizing different aspects of their kit to give them a different gameplan. It's typical to have two all-around ones - one aggressive and the other defensive - and then have any extra ones past that pushed more towards extremes with altered overall character properties like health, size, etc. Roy kinda fits that role in Smash - his sweetspots are up close and he's a fast-faller rather than a semi-floaty.
This raises the question of
how we specialize Lucina in a slightly more aggressive direction, and what aspects of her and Marth's shared kit should be emphasized for her, and de-emphasized for him, and feels like it fits her as a character?
Personally, considering the difference in their posture while charging shield breaker (Marth's more elegant posture vs. Lucina's
complete lack of chill) and the key differences in their backstories backstories in their home games and the circumstances behind them entering the main conflicts of their games, I'd definitely emphasize the idea that Lucina's fighting style is less refined and that she has poor form. Maybe take come cues from Forsburn and Clairen in Rivals of Aether.
The biggest change I'd suggest is either changing the way Dancing Blade works and what it's used for, or even replacing it entirely. I'd also consider maybe changing the way her counter works - possibly something more Focus Attack-esque? Or just making the Iai Counter custom from Smash 4 the default.