We need to invent the idea of well-formed bills, with requirements of clarity, factorability and maybe focus. This probably means we need a new branch of the judiciary to adjudicate whether bills and edits thereto are well-formed before they can be enacted. This would allow you to make a precise definition of a line-item, and therefore a line-item veto, without allowing monstrosities like in the article, among other benefits.
Edit: alternatively, just don't allow bills that do more than one thing.
It would be nice to have judicial review of all bills as they pass, for constitutionality as well.
Some would argue that it would slow down the legislative process. But looking at the sheer size of the US Code, I would argue that it's not at all a bad thing.
I'm afraid you'd probably need a constitutional amendment for that. And it might not be easier than the "getting money out of politics" one (https://wolf-pac.com/the_solution/).
Oh absolutely. Honestly it would probably be more effective to implement voting reform (single transferrable vote FTW) if you're going to the bother of a constitutional amendment.
As for your link, I can't even figure out what their proposed text is.
They actually used to have the text of an amendment. Now their website is more flashy but with less content. Too bad. It's about non-personhood of corporations and restrictions of corporate donation at the minimum, and at the maximum something like publicly-funded elections.
Strangely, the people that say this don't tend to hold out California as a model of government that the federal government should emulate, despite California actually having an enforced single subject rule.
In fact, they tend to accuse California of being worse than the federal government, but also at the specific things that they attribute to the lack of a single subject rule.
Keep in mind that a political party is larger than any one person.
Both major parties in the US benefit from maintaining the status quo. Neither one wants to do anything that takes power away from Congress and puts it back in the hands of the people.
Whichever party is in power during any given cycle is the one that benefits from gerrymandering, omnibus bills, fillibusters, etc. So why would they change something that's useful for them?
Then when the pendulum swings to the other side of the aisle, the situation still holds true. Party A and Party B will fight each other to the death, but neither one wants a Party C to rise to prominence.
Edit: alternatively, just don't allow bills that do more than one thing.