1. Make better UI for existing options, like /droploot command. I am tired of trying to explain to people I try to rob what I want them to do. Most just stare at me emptily for a minute and then try to run away, which gets them (and their horse) killed. People should never have to use chat commands (that are also not written anywhere, it seems) for gameplay actions.
2. Add options to force robbing. I want to be able to check the inventory of other players and their horses and take things from there against their will. Perhaps should require some kind of mercy mode extension or something. Maybe upcoming stealing will do that, but from what I read, you won't be able to take everything from your target, so it is not exactly that.
3. As a reasonable implementation of the previous point (#2), make the button "Surrender" (I think it is called that) in mercy mode that actually opens up you to being looted (including your horse inventory) instead of suiciding. By the way, when I was first put into the mercy mode, I actually was surprised that the button killed me instead.
All the options above make robbing people more intuitive and pleasant for everyone involved and will make killing horses unnecessary. Currently you can't really check what is inside the horse and take it without killing it. And I don't think the developers want to encourage killing horses, even though currently pretty much every mechanic just screams "kill the horse first".