Continuing with the ideas of allowing the client to request only a subset of the elements contained in each item in the feed, and of allowing the client to request only new and/or updated items, the next idea is to allow the client to request one set of elements from new and/or updated items, and a different set of elements from old items. This could be useful, for example, to enable display of new items while tracking which old items (cached by the client) still exist in the feed, and which are gone.

The same thing could be accomplished with two requests--one for new items, and one for a uniquely identifying field (guid, for example) from everything in the feed. But combining the two into one request would reduce overhead by requiring only one request.

Finally, another variant of this idea that should definitely be optional to support would be to enable requests for only the changed elements of updated items (along with a unique identifier). For example, if a headline ("title") is updated, the server could send just the guid and title for that item, and if in another item, the description is updated, send the guid and description for that one. This would require the server to keep track of the modification time for each element of each item, which is why I would definitely make it optional, but who knows--somebody may want to support such a function.