The map of challenge caches was primarily designed for the assistance of challenge writers. But I do agree adding the standard set of filters would be nice.
You can achieve a similar objective using Tools > Map based > Live map. Filter in just the unknown caches, add "challenge" in the cache name and exclude my finds. Of course, this map was not designed for this kind of work so is a little slow if given a large area. It won't show you new challenges since a date, but it will show you challenges you have yet to find, probably a more useful piece of information. If you are a paid member it will also show you if you already qualify for the challenge.
Another solution that will do exactly as you request if you are a paid member is to use Statistics > Profile > Map compare. Leave compare with empty, tick none found. Add filters: cache name contains "challenge", cache type unknown, from/to hidden date as required. Faster than the above method but requires more setup.