If anything, I feel like a non-royal survivor of the serenes massicre would be interesting.
As for a new idea, I've had an idea for a MU in tellius for a while.
the idea is that the MU is a dain conscript and one of the few survivors of the clash between the dain army and elincia's escort. the MU bascially decides, fuck this, when him/her and a few others find elincia and decides to protect her instead, and then you get intergrated into the GM.
Being former dain would make for interesting supports and dialogue (with bosses as well as commentary on the situation) as well as insight into daein as a whole.
In PoR's epilogue, you'd have a choice to where you'll go, somewhat pending on your supports, which will determine where you are in RD:
Daein(the defualt option): you go back to daein, your home. you join the DB early part 1.
GM: have an A support with any of the original members of the GM+mia(I find it funny that she's the only permanent join from PoR if what we see in RD is correct)
you'll join ike's party from the start
Gallia: have an A support with a laguz, and act as a beorc representitve in gallia. You'll join with lethe and mordy in part 2.
Crimea: have an A support with any crimean knight or charecter. if its Brom or nephnee, you'll start on 2-1 with them. otherwise, you'll join with geoffery in 2-2.
Tormod: A support tormod or muraim and you'll join with his crew in part 1. you won't bail the DB though as daein is still your home country.
Begnion: support tanith and you'll join when she does.