Just an idea that I've been thinking about for a little bit... we've obviously seen a number of popular characters from across Ylisse, Hoshido, Nohr, and a few other places setting aside their armor (or in some cases, at least some of it) in favor of special outfits fit for the seasons. In a number of cases, these heroes also wielded completely different weapons from their usual, or boggled themselves down/obtained a mount that further set them apart from their base forms, even giving up their practical or even legendary weapons for stylish or downright silly weapons... usually.
However, what I was thinking... what if they still switched outfits, but were still the same unit? Say, for instance, Marth were to replace his modern armor with that of his classic Shadow Dragon and the Blade of Light armor? You know, the one without pants? Or if Anna were to borrow her otherworldly sisters Trickster outfit from Awakening? Or take literally anyone who started unpromoted in their original game, and then "promote" them? If so wanted, they could even alter the poses of the unit in question, since a lot, if not all of the early units have somewhat generic battle ready poses (I think Takumi had the only "unique" Bow pose at launch...)
Simply put, they're not official alts that have different stats, movement type, or weapon, simply the same unit wearing something else. Like an accessory for the entire body. Think the costumes seen in FE: Warriors.
What do you think? Would you want such a feature? If you do, what kinds of costumes would you want, and for who? These costumes don't need to replace seasonal alts, heck they don't even need to alter the units artwork. They'd just be fun customization pieces with no benefits besides looking cool.