Novalis proposes an RSS mix-tape system:

What I think the world needs is an RSS mix-tape system. So, you pick your favorite 100 blogs, and they all show up on one massive page, each entry with a checkbox next to it. You check the checkbox, and it comes out on an output RSS page, which is essentially your "mix tape" of the input feeds.
...
The input RSS feeds are the albums you're picking songs off of, and the output RSS is your mix tape. A traditional RSS aggregator is like simply playing each song off each CD. What I propose that lazyweb create for me is more selective -- "this is what I found interesting among these twenty blogs."

He also mentions the idea of 'remixing' two or more 'mixes'. I think AmphetaDesk could be hacked to do something like this.