One thing I think might be interesting.. a 'Party Based' Senate representation. Leave the House the same as it is- population based, majority wins. Switch the Senate to be representative based on major parties in each state.
Example:
State A has large proportions of Dems and Reps. They get two seats - one Dem, one Rep.
State B has Dems, Reps, and Libertarians - great. one of each.
State C has Dems, Reps, Libertarians, Greens - sure.
You can work out 'large' by saying that if a party has at least ( 100% of population / # of parties in state ) members, then it can get an official Senate seat, to keep the parties with only 2 people in them from having an entire say.
Just an idea... if we were starting completely from scratch..
And of course, only the party can vote for their own.