Oooof, so many, we have boxes full of prototypes and other ideas. We tried Debit Card and PoS, we were very successful being Bitgo before Bitgo, as backend to companies like BitMex and Coinapult but holding ppl's bags was not our dream in life so we closed that down. We designed a COLDCARD much earlier but never made it. We even released but never launched a web file selling site using bitcoin. There was also a CloudFlare replacement "CloudFire" with in browser proof of work, but that never went anywhere. That's at least what comes to mind right away...