I'd like to add something to your list of nice-to-have features: A category mapping table.
The calendars I'm sharing feeds with have similar - but not equivalent - category names. I'd like to be able to set things up so that events flowing in get auto mapped to the categories in *my* calendar of my choosing. For instance:
My calendar has categories:
Another calendar that feeds us has categories:
Workshops & Courses
Obvious to the eye that they're pretty darn close, but of course to our computers they're not the same. What's happening now is that the incoming feeds are creating new categories that didn't previously exist. If a visitor goes to an event that is in a category from the first list, clicks on on the category link there, he or she won't see any of the events on the (nearly the same) topic on the fed-in list.
What I'd rather have happen is that their "Straw Bale" gets changed to "Strawbale" as it feeds in, without my having to manually reset them every time. It'll help keep down the category clutter too.
Ideally in the admin panel we'd be able to see them grouped by incoming feed.
It would also help to have the ability to feed many different incoming categories to a single category on the target site, for instance:
earthen plaster >>>> all feed to "Plastering"
and vice versa, if the feed is less specific to be able to add more than one category on the target site:
Workshops & Courses >>>> maps to both "Workshops" and "Courses"