I personally don't want avatars to return especially as a main character, avatars are the exact opposite of what a main character should be:
A good main characters should have a design simple, yet recognizable that makes him/her stand out from others, every aspect of his appearance (hair style/color, clothes, size etc...) are especially chosen to express something about the character (Fire Emblem is inspired by medieval Europe and in Europe, blue is considered as a color of royalty that's why so many main lords have blue hair/clothes for example) while an avatar is customizable and players mostly give him/her a certain physical attributes just because they find it cool/beautiful, which makes the character way less charismatic (that's why you never have a customized character on a video game's cover art).
Main characters also have their own personality traits or at least have the classic-shonen-protagonist personality while avatars tend to have a bland personality made to correspond to most people using the Barnum effect
So having an avatar as a main character is basically following the story of a bland uncharismatic character that I'm supposed to relate to and I don't wan't that for FE 16, creating a good main character is really difficult and when a game choose to make an avatar instead, I often see it as: "We were too lazy to make a main character, make it yourself"