I've thought about this as well. On the one hand, you have a system that assumes one size fits all for two specific parties. That will never be the case. I can guarantee you there's people who will disagree with both parties on any number of things.

On the other hand, if both parties stopped behaving like the other is an enema of the state and actually attempted to collaborate for the good of the American public, that would be great.

I think allowing other parties would be useful, the problem is even though other parties totally can be voted in, most people don't bother because you could lose your vote altogether if the party you disagree the most with wins. To put the example more bluntly I heard it said that Obama said a vote that isn't for Hillary is a vote to Trump, as in, if you vote third party, you may regret it.

Personally, I don't think two single political parties can represent all the hopes and dreams of all 300 million citizens. I know this may sound controversial to some, but it is starting to prove itself to be a problem.