Dark Link works far better as just a palette swap because he's ultimately meant to be an exact replica of Link.
The alternative would be to make him like Daisy, an echo fighter whose differences are only visual. And at the time of Melee, some actual effort went on the clones to make them distinct so something like that would have never happened back then.
They aren't only visual, though. We know for a fact she has slightly different data that affects her animations. Like, they're terribly done, but she was never 1:1 to Peach. It's more like she's 99%(probably more like 99.99999%, but bear with me on the point) the same.
The problem is that they're so slightly it'll never come up in any kind of match, so people mistake her for being a pure visual change. However, we do have recorded data of the gameplay differences. It's just simply her animations have barely different hurtboxes(but still actually different). It's really the only thing that makes her impossible to be just a costume. She has gameplay changes(poor ones in total), not unlike Lucina in 4. Costumes aren't allowed any kind of actual gameplay difference, no matter how irrelevant it seems. That's the rule Sakurai made clear to us a long time back.
That said, it's not hard to make Dark Link a low effort Echo with barely any differences(Richter suffers similarly, though I only remember his holy water being different. His taunts might have minorly different hurtboxes, though, putting him rather on par with Daisy's changes. To give credit, Dark Samus also has more elemental changes, and a few minor hurtbox changes too. She's above both, but only barely).
But back to Dark Link; he has actual unique items in various games, and can do anything Link can do sometimes. Though that said, Link's Shadow(Zelda II), Dark Link(OOT), and Shadow Link(FS) are actually different characters, but effectively there's no tangible differences besides stuff like lore. So you could easily make it an Echo of any of the three without issues. This is likely why they're just simple costumes. It happens that all three Links at one point had a shadowy counterpart anyway.
Ah, so I was wrong on the FE characters. Still, there's precedent for setting up placeholders for new Pokemon games; FE has also been singled out for promotional characters, so I don't think it's too much of a stretch to predict an upcoming Fire Emblem game being given a reserve slot
Pokemon's a special case where it's a multimedia juggernaut that it's odd to not have a new one since the franchise is guaranteed works almost every year. It's pretty difficult to not find something plausible from the latest work, even if it's just for a Spirit/Pokeball. Fire Emblem isn't in the same boat at all. It hasn't really had special situations beyond "easy clone types" at this point. And even the two times it happened with Pokemon, both also had anime popularity at the time(Jigglypuff was big, and Pichu had its own movies) that made the choices even easier. Pichu also doubled as the mascot of the Baby Pokemon, so it had another good reason to be chosen. ...When you're the pre-evolution of the franchise's mascot, you tend to get some attention in other ways, heh.