If I were to add a Fire Emblem Heroes character, I'd pick Anna. She's a notable recurring character in the Fire Emblem series, so she doesn't and won't have the issue of being old and irrelevant like anyone from the Tellius series and backward*, she wields an axe in this game, so she wouldn't be able to be the target of anime swordsman haters (though there will still be hatred to her 'cuz Fire Emblem), and her roll as a shopkeep could lead to some creative ideas.
For example, Anna loves money right? So when you hit your opponents, money will pop out of them, and your goal is to collect it, just like in Coin Smash. For an extra cute touch, the type of currency can vary depending on the series your victim belongs to (Coins for Super Mario Bros., Rupees for The Legend of Zelda, Rings for Sonic the Hedgehog, etc.). And just like in Coin Smash, getting KOed will cause you to lose money. Now comes the fun part: When you collect enough coins, you can summon another Anna, and view her wares. Provided you have enough money, you can buy any one of her three items. Each Anna will have a cheap item that's only marginally useful (like a green shell or a boomerang), a medium priced item that's decently useful (like a Killing Edge or a Home Run Bat), and an expensive item that is very useful (Like a Bob-omb or a healing sprout). The Annas' stock will be different each time you summon one, so you can save up a lot of cash for a good item, but you will never know which selection you will get.
EDIT: I just realized this is literally just Shovel Knight's gimmick in Rivals of Aether...
*meaning, side games where they bring back everyone and remakes aren't the only time you'll see her moving forward.