I could see this becoming an MMO with half the people being the opponents and the other half being the towers or whatever. The opponents would have a similar view to that of a racing game, but there would also be AI driven opponents to compensate for not many players. If an opponent were to die, he would take the nearest AI opponent availible. If RPG elements were also added, I would play this game all the time. If the game took this direction, I guarentee that it would be a big hit. Just don't forget the RPG elements.
You Should Be able to stretch the quarter notes notes into a longer note, that take more space, but are held longer. you would just grab the right end, and stretch it right up to four notes long, Thus allowing longer notes, and more possibilities. + If you agree.