It'd be pretty sweet if we had an emo male pegasus rider flying on a black pegasi. That would rock!
And come on, we need more color variation in all ridable animals. Horses don't come in just brown, white and black, they can have spots, patterns, anything goes here. Spotted horses would work great for farmers like Meg and Brom.
However, I'd rather see some more human races represented in the series than all units having characters from both genders.
It gets very cliche seeing a bunch of anime white guys going at it. Why not play as a totally diverse army instead? That would be totally unique and entertaining.