PlaYeRaNoN

I don't think they'd add one in dlc. And I was pretty happy one wasn't in Echoes.

i was never much a fan of avatars, to be frank. But probably more because Kris ruined the idea and Corrin  only bitteres the taste. Robin would have been good enough as his own character and Mark was such none person that I forgot Mark was a thing. Ehehe....

i feel if they're gonna do an avatar, it should be something like, another villager friend of Alm that adventures with him, but still keep Alm and Celica as the leads that take center stage in the plot. Maybe have them be a side character that chimes in like Tobin and Gray do for Alm, or like what Mae and Boey do for Celica

Yes, that's what I hear most of the time. I think it's probably the best way to go about a MU.

They can't be too important since it can  ruin the story (as seen with Corrin, Kris, and arguably Robin, taking the spotlight from Chrom in the end) as they usually act as a black hole that bends the story and other characters to be in the spotlight . They can't be too insignificant, else they'll lack agency and there won't be much use for them. This is sometimes criticized with Mark. 

So in this kind of scenario then it's more a question of stricking a balance when it comes to plot involvement and relevance. Like making them some supports but not too much and giving them a role in the story but nothing too big: basically a side character, just like you said. 

Yeah, side character sounds perfect for them.

Where are you in Echoes?