Just last night, I thought of something that would make for a great 50th anniversary set, though it'd probably appeal to the hardcore fans a lot more than casual audiences (if that matters): A disc or set containing 50 inserts or street scenes that haven't been seen in years. Meaning ones that don't have images on the wiki.
Not sure how aware the people at Sesame Workshop are of what's rare or not (well, I'd expect Joe from Tough Pigs to be aware), but anybody compiling such a release could check the Muppet Wiki sandbox page for unidentified segments, listing inserts that the wiki currently doesn't have images for, not to mention looking at the list of Sesame Street episodes page that has a way to identify if an episode has a full guide, a full guide without images, a page but not a full guide, and if there's currently no page on the wiki. Those would be good research tools for determining rare segments/street scenes to include (and if they were to focus on 50 but include street stories, would they count all street scenes as one or multiple segments?).
Of course even if they were to compile such a special release and go through the effort to see what segments don't have images on the wiki, who's to say that some of the segments chosen wouldn't somehow surface online by the time such a release came out?
Two other, somewhat niche ideas I thought of on a similar concept, would be a release of 50 rare Big Bird clips and 50 rare Oscar the Grouch clips, be they street scenes, street stories, or inserts that don't have any images online.