The idea was outlined in my "Anyone but Leo X": Instead of going south to Naples and winding up in Spain and then getting killed in Navarre, Cesare heads to France. There he reunites with Charlotte d'Albret, meets his daughter, Luisa (who IIRC never met her dad OTL), and because Charlotte hasn't contracted syphilis from him, a son (not sure which of the common Borgia names he'd get: Alessandro, Rodrigo or Giovanni) is born a few months later. Although to be fair, it'll be his parents last child (couple of stillbirths or miscarriages thereafter, but Luisa and her brother will be the only kids of Il Valentino).
Now, history progresses more or less as OTL, Julius II goes to war against Venice, and invites the whole of Europe to dogpile in TTL's version of the War of the League of Cambrai. However, similar to OTL, Julius realizes this was a bad idea to invite the French in, because in their train is none other than Cesare Borgia (whom Julius screwed over royally).
Can Cesare stage a comeback as duke of Urbino/Romandiola? Will him surviving (I figure that with two parents who both made their seventies, as long as he avoids dying unnaturally he should be fine until the late-1510s, when the death of Lucrezia will hit him hard and he'll probably follow not long after) change much? Will he get involved in the papal conclave when Julius II dies? Or even at the Council of Pisa? Who might he marry his son to (the Borgia pissed off most of the native Italian families, but if Cesare wants his dynasty to last Italy, his foreign-born son to two non-Italian parents can't marry a foreigner)? I figure his daughter likely stays engaged to the duke of Mantua unless a better option comes along (who?). What might the effects of a lasting Borgia state in the Romagna be? - with the war of Cambrai, by my understanding, Venice supported the rulers of Imola, Cesena etc, while the pope essentially wanted them directly under his control (much like Cesare had had). So, here, would Venice back Cesare? Or the petty rulers?
And since this isn't going to be a Borgia-wank, the lands Cesare is after are only the duchy of Urbino and the lands of the Romagna where he ruled, so no Tuscany, and Piombino goes back to the Appiani.