So just a minor theory but in the FeMU battle description, they talk about the next grand hero battle. I have reason to believe that it will be Micaiah from Fire Emblem: Radiant Dawn. I have a couple reasons to support this. (It's just a theory so take it with a grain of salt.)
•FeMU has B tomebreaker and it was stated that she would be useful in the next grand hero battle. Since Micaiah uses Light magic I'm assuming this groups her in the Thunder Magic group with Linde, Owen, and MaMU.
•It has been approximately 3 weeks since Fire Emblem Heroes' release and yet we have had no note of the popular characters yet. Due to the timing possibly with the switch release, I believe Fire Emblem Heroes will release the new focus summon (Likely Path of Radiance/Radiant Dawn) along with the popular character ballot. As it so happens, Micaiah coincides with both of these.
•Of all the most popular Fire Emblem games that have been represented in Fire Emblem Heroes, Radiant Dawn and Path of Radiance are the only ones to not show up. (I'm only referring to the highest sales numbers across Japan and the USA, of course these numbers vary across Europe, but this is a generalization.) this makes it so the chance of representation from Path of Radiance and Radiant Dawn is high.