So I've been toying lately with an idea about having the Pazzi Conspiracy fail more dramatically than OTL.
In 1478, a conspiracy involving the Pazzi family, the Archbishop of Florence, possibly the Pope, and a cast of thousands tried to assassinate Lorenzo and Giuliano de Medici. Lorenzo survived by locking himself in the sacristy; Giuliano was butchered on the cathedral floor. Revenge was anticlimactic - the conspirators had seriously overestimated their popular support within Florence, and the mob had killed most of them before Lorenzo got around to it. He went on to rule ably alone, earning the sobriquet 'the Magnificent'.
But I want to have Giuliano survive somehow, so that the brother's partnership can continue. Giuliano did all of the baby-kissing, speechifying and general public relations hoo-ha while Lorenzo handled the money and supervised the civil service. With his brother still handling the circuses, Lorenzo can contnue concentrating on the bread - and notice one of his subjects, a 26-year-old Leonardo da Vinci and get him a formal education and perhaps a government sinecure. Yes, it will be a bit of engineering wank most likely.