I can only imagine the can of worms that would open up, as litigation on top of litigation involves the courts inexorably in determining what is and is not related to the subject of the bill, because almost anything can be considered "related" somehow. Just look at the commerce clause, and what Congress gets away with regulating because it's somehow "related" to interstate commerce.
If you really want to make a constitutional change that would help with this situation, probably the best thing would be to limit Congress's ability to raise money in the first place. Raising taxes should require a supermajority, and they should automatically decrease over time unless bumped back up periodically by said supermajority.
The Confederate Constitution required that ALL appropriations bills pass by two-thirds majority, limited spending to 'provide for the common defense, and carry on the Government', and provided that 'no bounties shall be granted from the Treasury' - i.e no Big Dig. Instead of taxes, usage fees would allow the people, and the laws of supply and demand to regulate the economy and federal government/spending.