What is a failed project that deserves another shot?

What do you mean? One of my failed projects, or one that I like that failed? And another shot by the person who created it in the first place, or are you looking for ideas of something to build?

OSSIC X headphones. fitting automatic HRTF generation in consumer headphones is something i'm surprised no one else has tried yet.

Google Wave, there were some features that were amazing like splitting messages in their own threads to branch off some ideas and keep conversations on track.