I fully agree with anything that would make sorting through (large) collections easier.
Dreaming big here because I know zero about coding, but icons for completed and/or unwanted sets AND only showing duplicates for completed sets (when looking at someone else's collection) would be absolutely perfect. In that vein, being able to block specific sets from being offered as a whole sounds great.
I don't mean something convoluted like, "Oh, I will only accept sets X, Y and Z for this particular one", but more general and focused on cleaning up the list in a trade window. So, say, I don't want set A, EVER, I'd just block it, and it wouldn't even show up in someone's trade offer window... if that makes sense.
Or just being able to remove completed sets from that list. Honestly, ANYTHING to be able to clean up / make it easier to make sense of collections, both your own and someone else's. Again, I truly don't know ANYTHING about coding and what is and is not possible, lol, so possibly I'm just throwing useless ideas out there But I like dreaming, so, who knows.