I occasionally visit Dave Winer's blog, as he has been known to have good ideas. One of these from a few years ago, that he's talking about again, was 'Reading Lists', whereby (in principle) you can subscribe to a list of feeds. When the list changes, your aggregator (in principle) can subscribe/unsubscribe to the individual feeds in the list, showing the contents of the listed feeds probably grouped together in some fashion. Neat idea, but it doesn't really seem to have caught on.
There are two de facto standards for expressing lists of feeds: OPML and RDF ("foafrolls"). The former is probably better supported in desktop aggregators, the latter maybe more visible in the online Planet aggregators (including Planet RDF, though that uses chumpologica/Redland rather than PlanetPlanet). OPML is Dave Winer's 'outline processor' markup language, for lists of feeds it has typed links. The RDF version uses the FOAF, DC and RSS 1.0 vocabs (very typed links). Away from the feed list application, the OPML format is usable in Dave Winer's outliner, and any RDF tool can make sense of the RDF (naturally :) but I reckon it does rather lend itself to FOAFishness - feeds are associated with a foaf:Person (and/or foaf:Agent) with a foaf:weblog etc. (I dunno, the domain is right on top of SIOC too, maybe some info using those terms could be added to the feedlists..?).
For any kind of Information/Knowledge Manager kind of tool (Personal or otherwise) built with RDF, it seems quite natural to periodically refresh data (not only feeds but pretty much anything in the domain of interest - FOAF Profiles probably being ubiquitous), so Reading Lists would sit comfortably alongside other features.
But in the 'simple' world of RSS, subscribing to feedlists is something of a complication. For instance, in the good Mr. Winer's latest incarnation, he's got aggregated pages (e.g. daveriver.scripting.com) not unlike those of the Planets, with an autodiscovery link in the HTML pointing to the feedlist:
<link rel="alternate" type="application/rss+xml" title="OPML" href="index.opml" />
OPML is RSS? I don't think even the Universal Feed Parser is that liberal. The kludge does get Firefox to show the target as a subscribable link, but then that's still not much good if the tools don't know what to do with it. But it seems to me there's a much simpler approach - use RSS. To get myself some markup to show I just bookmarked this blog's feed with del.icio.us and had a look at the feed that produced, and it contains (trimmed) this:
<title>Danny Ayers : Raw Blog (feed)</title>
Now a current aggregator would see that and probably just display it as a HTML-style titled link. But if the aggregator bothered to do a HTTP HEAD, it'd see:
To a (non-RDF savvy) aggregator that means an RSS 1.0 feed. So, aggregator dude, subscribe to it. Atom <link> elements have a (mime) type attribute, so there the HEAD wouldn't even be necessary.
While most feeds are a changing, fixed-length FILO queue of entries, there's nothing to stop them being a variable length list.
In other words, the simplest RSS feed list is an RSS feed. Even if the aggregator needs a little help in recognising a feed list, it's got to be easier than understanding an entirely different format (published with an inappropriate media type).
Ok, so personally I'd go straight down the RDF route, it's a heck of a lot more flexible. But an RSS-format Reading List does seem like low-hanging fruit for non-RDF tools.
Anyhow, if anyone's building an aggregator (they're a great little starter app when learning a new language), consider Reading Lists as a feature.