That’s a thought, though that’ll also introduce some new political strategies that one might not want, like making poison-pilling legislation a much-more-powerful move or immediately proposing and voting down legislation just before a given legislature departs to kill the ability of the incoming legislators to pass that legislation.

It also may be hard to draw that “substantial enough” line. Similar problem to determining what qualifies as a rider for the purposes of anti-rider restrictions (“you can’t just attach unrelated legislation to legislation” and “well, what qualifies as unrelated?”).