In the future, everyone will want to build and republish custom webfeeds from existing feeds for all kinds of purposes. (This is RobinGood?'s NewsMaster? vision.) An ecology of services will emerge to help them do exactly that.
A number of operations on feeds can be envisioned. Taking a cue from the operations of set theory we could for instance define the following:
Implementations of FeedSplicing services have begun appearing in 2003:
Also of interest may be two feeds automatically splicing themselves together, so whichever is read, both are seen. (e.g. UnifiedClusters)
An example of splicing follows. This is the merger of feeds from some CommunityWiki members. (Seems like the feed is flaky, so don't hold your breath..)
http://blogdigger.com/groups/rss.jsp?id=173 returned no data, or LWP::UserAgent is not available.
The subscriptions lists is editable from this page.