So it bothers me that Felix and Dedue are the only rival/retainer(right hand) pair that don't have a paired ending so I was thinking about what it'd be like.
So first off they'd need another scene. I believe their current B conversation would work best as their A conversation so there would need to be a new B conversation. I think something that brings up the Tragedy of Duscur would work best. Maybe its just my desperation to hear more about Glenn but I'd like to see the two have a conversation about him and about how Felix's "hatred" for Dedue does not come from blame for Glenn's death but from his anger with Dimitri and his hatred of blind obedience (now that i think about it perhaps part of what I'm saying could be an A conversation)
Now what's left is a paired ending
After the war's end, Felix took up his late father's position at the King's right hand and with Dedue became Dimitri's sword and shield. While the Sword and Shield of Faerghus did not share many words their deep trust and friendship was undeniable. Upon Dimitri's eventual death, the two took up post guarding his grave and watching over his sons. In each other they found solace for their king and friend's loss until the end of their days.