An amendment which made the number of Senators from each state variable (based on population, GNP, number of movie stars in residence, or any other basis) would clearly be void from its inception unless either (1) it was ratified unanimously, or (2) every state which did not ratify it would have at least 1/(number of states) of the Senators unless or until such time as they ratified it.
The existing Seventeenth Amendment is perhaps questionable for much the same reason, since there are IIRC three states that were present at its ratification but have not ratified it themselves (any state that ratified the Seventeenth presumably consented to it; likewise any state that joined the Union after the Seventeenth was in force). Of course, even if the legislatures of those three states were found to have the power to appoint Senators themselves independent of the popular vote, I doubt any of them would want to do so.
The Constitution specifically sets out how it may be amended, and it doesn't require unanimous vote. Once the original parties agreed to that voting mechanism (which they did by ratifying the Constitution), that's what is in effect, and it's legally and constitutionally binding.