In many 'ultimate team' games including FIFA and Madden, there is an action house which allows players to put up any character or player which s/he may not want, allowing others to bid on it using their hard earned in game earned currency. Obviously, depending on the character's effectiveness, there may be price differentials. These games also have the 'summoning' feature and still make a lot off of them because of many players who struggle to make enough in game currency to pay for the characters they want in the auctions. Such a system allows for those who work hard in game to get better players, but still gives a reason to purchase 'orbs' to try and get characters quicker.
Obviously, for Fire Emblem Heroes, I don't know exactly if people would be interested in an Auction House or a way to trade characters. The game already has implemented feathers to be the in game currency, so it wouldn't be hard to simply judge a player's effectiveness in completing set tasks just like in arena mode and give out feathers accordingly.
For those who are F2P, it allows for greater immersion within the game. It allows you to grind, and then hopefully one day get that Summer Corrin (for me) or Ike or Bride Cordelia you've always wanted. Would this system work? I don't foresee Nintendo ever doing something like this but just thought it would be interesting to speculate on a cool idea. Would you like such a system to be brought into the game?