I know the prompt was "If Smash came out for the first time today with a twelve character roster," but even assuming all other non-Smash games still existed and everything played out exactly the same way, Marth would almost certainly be the Fire Emblem character picked for it.
He's popular, iconic, and the face of the franchise (yes, more than Anna is). Four main series titles with him as the main protagonist, one where he's an important background lore character, the most recent one where he's an important supporting character, and a character with six versions of himself in Heroes. His existing popularity is what got him into Melee to begin with, and more recently, the backlash from fans during CYL2 literally changed how Choose Your Legends tallying was done from CYL3 onwards. He's the Fire Emblem Lord.
Even ignoring the absolutely ridiculous claim that you can't be a main character if you die in the story (especially considering Marth ISN'T EVEN DEAD in Engage's timeline; the game takes place in an entirely different universe and all of the Emblems are manifestations of Heroes from other worlds, so the only one who's explicitly postmortem is Sigurd), Marth's relevance never even faded.
Literally the only way he wouldn't be the one picked is if they went for one of the House Leaders, simply due to just how popular those three are over literally every other character in the franchise, but even then Marth would be the pick nine times out of ten. And even then, one could always get in as a second character, which wouldn't be out of the question (but that's a talk for another time).
