imo, New Mystery of the Emblem actually helps Robin's chances, in a way.
FE12 was the first game to give you a Tactician as a playable unit. Mark did have some role in the plot, but not much, but he/she was a unit you could style after yourself, and give you some control on how you wanted this unit to function.
FE13 took that concept and amplified it off the charts. Robin is given Deuteragonist status in the story with Chrom, Robin can be reclassed into everything but a Lord, Robin has 20 hair samples, 6 size samples, and 10 face samples to choose from, can marry any unit, and the story falls apart without Robin in the game. Robin is, by all accounts, the player, styled to blend in with the world of Awakening.
And I don't think it'll stop with Robin. FE14 and the rest, whenever they come out, will almost certainly have a customizable Avatar for the player to create and put into that game's world, especially since Awakening was so popular with newcomers, and they'll all be styled off of Robin's template in a way.
Robin is a viable candidate not just because of the moveset opportunities, the chance to change up typical FE Smash Bros. archetypes, and Robin's own purpose in Awakening. Robin represents a newly-introduced facet of gameplay for Fire Emblem, and one that was well-received to boot, and one that will almost certainly be in future FE games, should Smash Bros. players formally introduced to Fire Emblem through it decide they want to play some FE games.
Marth, Ike, and possibly Chrom are main characters. Robin is a legacy.