He'd planned a great statue of a horse, completed the mock up, but then the supply of bronze he'd planned to use was melted into armaments.
Preparations he'd done on a wall for an artistic creation washed away in a rain, and he never returned to the painting.
Even in his lifetime, one of his best works, The Last Supper, had started deteriorating. He'd chosen materials which worked better with his method of creation than ones which were more stable over time. Even if he knew this would happen, I'm sure he would have continued exactly as he did.
None of his masterpieces interest me as much as his sketches of imperfect faces, and the ones he'd exaggerated. (At least, I think they were exaggerated.)
His art and intellect caused him to have genius status. But when I discovered his handwriting style, it added a level of artistic flair. He sometimes wrote backwards, from right to left. This quirk dramatically increased his status on my list of "famous people from the past I'd like to have dinner with."