we need to return more power to the states (which gives us a check on the federal government).
Specifically what powers should the states have that are currently exclusive to the federal government? The way I see it is that the federal government enforces the bare minimum of an individual's rights, controls interstate commerce, and runs other projects that would be inefficient for individual states to run. Some examples of these would be freedom of speech, outlawing slavery, running the military, setting a currency, etc. There is nothing preventing a state from granting it's citizens greater rights then what the federal government ensures or providing their own military. You may disagree with how the federal government chooses to protect individual liberty or even what those rights are; but that's a very different discussion.
Also the states can amend the Constitution without involving the federal government, all they'd need to do is call a convention and then later vote on the new amendment. That's the greatest balance against federal government that there is.
Better system for single-winner elections. It should allow you to specify your primary choice, and also your backup choice(s) should your primary fail to gain enough support.
Who would enforce this? The federal government? I think we have single run off because of tradition more then anything else. There have even been state laws that split their electoral votes proportionately (I think they've all been abandoned now). As long as the federal government ensures that everyone can vote and that everyone's vote is equal I think it should be up to the states to determine what style of voting to use.
Proportional representation in Congress. If every district in the nation votes 50.1% for Party A, then Party A has 100% of the seats of Congress. Party B gets nothing, although 49.9% of the nation supports Party B.
That would be true if we were electing parties to congress but we're not, we're electing people. Representative X from Party B is not at all equivalent to representative Q from Party B. I realize that the political rhetoric tends to slant towards Party A vs Party B but I'd much rather vote for someone who fairly represents their constituents instead of a generic interchangeable member of a Party.
Electoral college reform. I don't know that we should go to a popular vote system for President, but the electoral college should at least force a proportional representation from each state. It should also remove the electors, and change to a simple count.
This will probably never happen because it would involve states relinquishing power and they'd never give permission for it. In the current set up states with a low population have a greater electoral collage vote per capita then a state with a large population. This is a balance so that large states can't steam roll over smaller states.
I'm not saying that our republic couldn't use some improvement but I think it's important to realize why it has worked so well. Like Churchill said:
"Democracy is the worst form of government, except for all those other forms that have been tried from time to time."