Just wanted to drop some thoughts on Lucina, after using her over 120 times online (With Friends and For Glory combined).
1. Foxtrot Dtilt is amazing
2. I don't know if most Lucina players use RAR SH Bair much but it's pretty good. I mainly use it when I'm trying to apply aggressive pressure, as it starts from below (which at least to me, makes it easier to approach with), deals more damage than Fair and hits the opponents at a better angle for followups. In particular, it's a good way to put Little Mac players in a bad position as it hits opponents more upwards and a flying Mac is a free Mac. SH Fair I use more for defensive pressure when I'm just trying to space moves without worrying about followups and the like. SH Nair is a decent option against "fatties", or when you just need a move that has its hitboxes stay out longer.
3. Her Fsmash is a very potent killer but I feel like I have to fish a little too hard for it sometimes. Best used to punish a landing or if you read how someone is going to get back onto the stage correctly, from my experience. SH Dair groundbounce can provide as a semi-effective setup for it but of course, your opponent can just tech it and punish you. Offstage Bair works to a degree. Other than those, my KOs primarily come from Usmash. Said Usmash is the one thing from Lucina that I feel is definitively better about her in comparison to Marth (granted I haven't played much as or against Marth anyway). She gets a nice slide when using it out of a dash/run and it's surprisingly easy to land, both by catching landing opponents and by just going straight at them with it.
4. Somewhat predictably, I think her best matchups are for the most part close range characters who she can just beat out with her sword. Toon Link and ZSS give me the most trouble, as approaching them is beyond just being a chore and requires heavy amounts of patience for something so unrewarding.
5. For that matter, she lacks true combos. According to the combo counter in Training Mode, Uair -> Utilt is a thing at around 10-40% on most if not all of the characters I tested (ie. Little Mac, Fox, Mario, Rosalina, Bowser and Samus) and she can get Uair and/or Bair off of Dthrow on the majority of the characters I tested it on when they're at 0%, though vectoring/DI plays a part in this so you'll have to predict where they'll go upon being thrown. These are when Lucina is at 0% herself btw, no Rage Effect here. Also on Fox Dthrow grants even more true combos (by what Training considers to be true combos, at least), including into Usmash and can continue to be a combo move up to at least 50% from what I've tested. Dthrow is a good move against him. Finally Uair seems to be capable of true comboing into more moves than just Utilt (including into itself!), I just need to test more. All of that said, she still can string together hits quite well, as opponents generally have just enough time to jump/airdodge your followup, in which case you just read what they'll do and punish them for it with the aerial they tried to escape from in the first place lol.
6. Dancing Blade is pretty useful as a damage racker. I generally try to keep going Up as they link together the best. When my opponent starts getting cheeky and tries to airdodge the 4th hit, I generally opt for 4th hit Down instead to punish said airdodge.
7. I feel like the lag and 3DS controls hurt her a bit, as they make precise spacing much more difficult. With how freaking ridiculous shields and rolls are in this game, that's a big deal. Spacing makes the difference between getting punished for every attack and applying safe pressure. Speaking of rolls, good luck trying to punish those without reads. Lucina has no reliable ways to deal with roll spammers. I find RAR Bairs are hard to do correctly because of the lag and controls as well.
I think Lucina's probably going to have very polarising matchups. Not going to try to predict her tier placement, but I'm doubting it's top. With all of the crazy camping characters that Newcomers R Us has brought us, Lucina needs more tools at a distance in order to be a primary threat, in my opinion.