Fire Emblem: Shadow Dragon / Mystery / New Mystery: Caeda

Why they’re here: If Marth is in because you have to put the first lord in, and Tiki’s DLC because they’re the first mascot (and a character with two canonical versions), then we’d have to include the first canonical love interest. Caeda’s role as a pegasus knight has been an archetype just as long as the blue haired lord and dragons, and she wouldn’t be far behind anyway.

What they could do: Given how she’s responsible for recruiting most of the units in her home game, she could “recruit” green or even red units to be controllable for a few turns. If they’re into her, of course (so a supermajority of men and a few women). If that’s not feasible, granting a “brave” effect (each hit in combat is actually two) via the “Triangle Attack” is a standard pegasus trick.

Other possibilities: Minerva (princess of Macedon) or Linde (most associated female mage) depending on if they’d want to have a melee or magic focus.

Fire Emblem: Gaiden / Echoes: Shadows of Valentia: Alm

Why they’re here: There are two story paths in Gaiden/SoV, and Celica’s in the base game so Alm is practically inevitable. One suspects the only things keeping him out of the base game were gender and weapon balance concerns, since the “oddly colored hair lord with a dragon-slaying sword” role is well occupied already between the Smash duo of Roy and Marth.

What they could do: Basically the same thing as Sigurd’s “Override” - charge through a row of allies, but with added effective damage on any draconic (or if they really want to be evil, mounted) foes in the way.

Other possibilities: Berkut, but their odds are the worst of all as he’s only been portrayed as a villain and they seem to be unwilling to use villains. Which is a shame, as we can never have enough Ian Sinclair.

Fire Emblem: Genealogy of the Holy War: Seliph

Why they’re here: Seliph has two advantages; as the son of Sigurd he’s not Lord Flambe at the end of Genealogy, and he’s more popular than his father (based on results from the annual Fire Emblem Heroes “Choose Your Legends” poll). This is probably the safest pick of anyone from an unlocalized game.

What they could do: Seliph is often associated with the Miracle ability - allows for random survival at 1HP - so a Miracle effect to allies within a certain range similar to the Essar Research Group battalion from Three Houses.

Other possibilities: Julia, Deirdre, Ayra - the two main heroines of the Genealogy generations and the game’s main myrmidon.

Fire Emblem: Thracia 776: Nanna

Why they’re here: Main love interest of Leif, the existing Thracia representative, who’s pretty good with a staff or a sword depending on your preference. There’s not really a lot of options here, though.

What they could do: The equivalent of a Fortify spell - mass healing, if not full screen - but since Thracia allows healing to fire twice if the healer is faster by more than 5 points of speed, the effect is doubled under those conditions.

Other possibilities; Reinhardt - this is strictly a meme pick, but Reinhardt is the main memelord of Fire Emblem Heroes and it’d be funny to go from “i no more money for you I now use the Reinhardt” to making him DLC.

Fire Emblem: The Binding Blade: Lilina

Why they’re here: Lilina is the one in Binding Blade who gets promoted midway through the game and carries the second half. Lilina is the one whose parent (in this case, her father) dies. Tell me again why Roy is considered the protagonist of Binding Blade when everything else points to Lilina actually being the one? (Oh, right, the sword.)

What they could do: She’s often associated with fire tomes, specifically Forblaze, so she could set up a defensive wall of fire for a few turns that does major damage to any foe fool enough to touch it. (Just… watch out for green units.)

Other possibilities: Binding Blade has two major dragons of relative popularity in Fae or Idunn, with the latter being less likely as the final boss. Though my own personal bias would be toward Igrene, the desert archer who is basically responsible for 99.5% of my Heroes completion.