No doubt people have said these, but I really would have liked more varied parent-child conversations and same-sex S rank conversations.
Specifically though, I would have liked to see something between Chrom and Cordelia. I would have paired them together if they had let me... I do like that because she has chosen to distance herself, but still...
The SpotPass characters (Gangrel, Walhart, etc.) should have some conversations, at least with Chrom as well as the Avatar. Not that I don't understand, but all the same. There's one SpotPass character he REALLY should talk to.