Rock Band has always been a console only series, but a crowd funded project by Harmonix can change that. If they can raise $1.5 million in 34 days, we'll get to see Rock Band 4 on the PC.
After 1 day, they are 11% towards their goal. Harmonix would leverage Steam and one of the nice things about that is they will allow you to generate and sell your own content for the game.
If you're worried about what instruments you can use, they state they will get most of the ones from Rock Band 1 to Rock Band 3 working. Most are Bluetooth so that's pretty easy and they might need to write drivers for the Xbox 360 ones. For Rock Band 4 instruments, they are working with Microsoft to make that happen.
I wouldn't mind see the game appear on the PC since my Xbox 360 died and I still have all the instruments in the basement. Unfortunately, any songs purchased on the console wouldn't carry over to the PC version, but that's understandable.
So we have a little over a month now to see if the goal will be reached. $45 will net you the game if you back it early with a retail price of $59.99 if it becomes a reality.