Article Five of the United States Constitution
Article Five also puts one absolute limit on Constitutional amendments.  It completely protects Section 3, Clause 1 of Article One.  This clause says that every state will have the same number of United States Senators.  It does not set a time limit on how long this clause is protected for.  It just says: "no state, without its consent, shall be deprived of its equal suffrage in the Senate."  As of 1808, when the protections on the other two clauses expired, this is the only limit that the Constitution puts on suggested amendments.  Any amendment, about anything else, could be suggested.