Maturity. yes, realism? Well it depends on what you mean.
I don't see a need to ditch the anime hair, magic, Pegasus knights, etc. or have a cast of entirely serious characters, but I don't think that's what you're getting at. In a perfect world I would have IS take some writing advice from the people who developed the civil war story arc from Skyrim. People found it mature and 'realistic' because it gave you two sides with real flaws and benefits, two equally respectable leaders for both sides of the conflict, as well as real reasons to support either side that you could see being actual important issues, if said world was real. The fact that people actually did debate over it as if it were serious business speaks to that. How often do you see an actual discussion about if Nohr or Hoshido is the better option based not on gameplay preference but lore?
My problem with the 3DS era of fire emblem is that it seems almost scared to take itself seriously. 90% of supports are about that characters gimmick that supposedly makes them endearing. (Awakening in particular suffers from this) 'Just look at them, they're all so quirky and lovable' says an increasingly smaller number of people as they start to prefer the 10% of serious moments that give the characters actual depth. Plenty of gritty things happen in these games but we don't notice because the game doesn't want to show us.