It struck me the other day while playing Rohan that Merry gets left behind on all of my Rohirrim Charges, since of course, he is without a horse. I was thinking it would be a cool feature if Merry could mount with Eowyn and provide some sort of bonus to her while doing so.
Alternatively, this could also replace or change her Disguise ability, since I know there is a lot of discussion on making this ability or more useful or different.