I doubt that the intent of the law was to have some over zealous game warden pulling balls out of people's guns and measuring if the patch on a round ball is too thick. I see it as a way to make sure that a plastic sabot is not uses.
I read it as any all lead conical and a patched ball will be considered to be the legal ammunition in the Gem State.
As far as pushing for something like an all traditional design, I don't think they will go that far. The state of Idaho still needs money from resident and non resident hunters wishing to pursue their game. Also, would this increase the mule deer population in that state? I get the impression that mule deer are declining all over the West. I read where mule deer aren't as adaptable as a whitetail. A whitetail can live in back yards and small woodlots. In fact, one of my favorite places is within 200 feet of a group of homes( I have the required permission to shoot within 500 feet of those occupied dwellings) . Could it be all the people on the West coast moving into and buying "racnhettes"? Also, let us not ignore the wolf issue in Idaho. Idaho got stuck with a Bill Clinton boondoggle with those imported Canadian wolves into Yellowstone and parts of the Gem State. Those animals can't live on mice and ground squirrels. I am willing to bet that a lot of mule deer ended up as wolf chow.
Also, getting back to the money end. I was at Thompson Center's Fox Ridge Custom shot looking at all of their outdoor wares. I chatted with one of the people whom I met before. He informed me that modern in-lines sell outsell traditional muzzleloaders ten to one. He also commented that once you get a traditional muzzleloader, most customers will keep it for life. With a modern one, many hunters want to trade up, so to speak. Money still talks and unless it adversely affects a game herd, I don't think a number of states will do what Idaho did.
I will predict that some of the Pacific NW states may follow but I don't expect places like New England, the Middle Atlantic States, the Southeast, Texas or the Great Lakes area to adopt such measures.