While I think it reasonable that if your skill increases you could make better items, there's a couple of things in the way of that. One, it would encourage people to spam their way to higher skills, which is something the Imms don't tend to like. Two, the more complicated things you gave for examples don't seem like something easily done on the coding end of things. Because of the troubles involved in implementing the ideas of mounts and ranged weapons, those and similar things aren't likely to ever get made, so I doubt siege weapons will either. At least, as actual coded objects. I suppose you could roleplay the presence of siege weapons all you like, but they (like mounts) aren't likely to really exist outside of room and mob descriptions.