For me what comes to mind is WestCot, the over engineered Mary Poppins flat ride for Epcot, the OG Muppets Courtyard that fell through after Henson passed away, the original Quinjet Avengers E ticket for DCA that was canned due to capacity logistical reasons, and finally the Play Pavillon.
But TBH, the list can go on of Disney projects that never saw the light of day for a vairety of reasons.