Defer the definition of solutions for aggregated feeds to a separate Internet-Draft that is not a part of the Atom core syntax specification.
Proposed (by [JamesSnell])
Aggregated feeds, while important, are currently not supported by the existing RSS mechanisms and are relatively rare in comparison to their single feed cousins. Given the guidelines for proposals set forth in this Wiki, this alone would justify moving the aggregation stuff off to a separate document, at least for now.
The 80/20 rule: If a feature will only be used by a small number of people, and will create extra work and headaches for everyone else, it probably doesn't belong in the core spec.
Pick stuff that's already been proven to work and be interoperable, and writing it down in a clean, clear way
Keep it simple: The simplest thing that can possibly work tends to be preferred over more complex solutions.
I absolutely acknowledge that there are a subset of folks for whom aggregated feeds are very important. But this is a subset. Let that subset document their ideas in a separate Internet-Draft; let them implement those ideas and build momentum for them; then let us later come back later and discuss the merits of merging those ideas into the core.
Lets Atom 1.0 get out the door faster.
Lets folks gain valuable implementation experience before committing to major changes to the Atom core spec to support what is currently an edge case
Keeps the Atom core simple