There was a similar mod that let you access horse's inventory in Oblivion, I find it weird it wasn't implemented. I guess its a bad idea since horses in Skyrim seem to think they're heroes and get themselves killed alot more. I think the ability to buy your own wagon you can travel with would be a very neat idea too and allow you carry massive loads of course. Maybe it can come with downsides, like higher chance to attract bandits or leaving it alone in the wilderness will get stuff stolen.