Now, one private notion I personally have, that I'd not advocating be made into law. One of those "It'd be an interesting idea..." things. Requiring an oath upon one's 18th birthday that they were an adult, tax payer and citizen. And a requirement to do something to earn the right to vote. Also, no federal taxes if you choose not to vote in federal elections.
It'd be too easy to be corrupted. Ueber rich would hire folks to vote for them via PACs or other dodges, and skip on the taxes. Poor would skip it and demand services anyways. Requirements would be corrupted so that the "requirement to do something" would become "public service" or whatnot, as in society is entitled to a portion of your life.
That's the problem with being a libertarian. You can't be a utopian idiot. You come up with grand ideas, shoot holes through it the size of Nebraska, and say screw it, why not leave folks alone?