The reason I oppose a Convention of States or a Constitutional Convention is because such an assembly could well be hijacked by leftists to establish a unitary national state, which would then be dominated by liberal big cities. That being said, the Constitution has been ineffective in controlling the growth of government power. The last century, and the first few years of this century, show the failure of the checks and balances.
The United States is too large a nation, and for better or worse, too ethnically diverse, to support a representative republic. The Founders envisioned the future of America as a great nation, but one with a common culture, based on the British heritage of the colonies, and a common religion, Protestant Christianity, though with tolerance for Catholics, Jews, and Unitarians. This is no longer true.
Size is another issue. America now has over 300 million souls within its boundaries. The average Congressional district has over three quarters of a million people. If we were to limit the size of a district to 50,000 people. we would have a House of Representatives of 6,000 members, clearly an unwieldy body. As currently existing, the House of Representatives can no longer realistically represent the general electorate. The passage of the 17th Amendment eliminated the role of the Senate as representing the legislatures and therefore the interests of the constituent states. It now serves as a House of Representatives writ large, with the only benefit being allowing small population states the same number of votes as the big ones.
I don't know of any long term solution to the breaking and diminishing of Federal power within the current framework of the republic. It may be a good thing if the West Coast states (excluding Alaska) or the Northeast and Middle Atlantic states (excluding Pennsylvania) were to secede, although the United States could no longer be as great a world power with the loss of these states.