IOTL Napoleon turned conquered European nations, especially Germany and Italy, but also other areas like Poland, into a hodgepodge of areas directly annexed to France with hardly a French in them (which fueled nationalistic resistance) and separate vassal states, some entrusted to various relatives and aides, some to cowed former enemies (almost all of whom eventually betrayed or failed him). This frustrated and alienated budding European nationalisms and helped turn them into an eventual enemy of the Napoleonic system. Other causes surely fostered the downfall of the Napoleonic system, but just as well Napoleon did not work to turn European national feeling into his ally nowhere as much as he could have.
What if Napoleon had turned himself into the champion of European nations, and given them the fulfillment of national unification, at the same time as he bound those new nations into larger imperial unity with France ? Say he does not invade Spain and focuses his resources towards destroying Austria and Prussia, dismantles all the ancient regime states between France and Russia, unifies Germany and Italy into unitary or federal states that are federated to France, sets Poland and Hungary up as independent vassals of the neo-Carolingian Empire, and declares himself the "Emperor of the French, Germans, and Italians", or even he claims the crown of Holy Roman Emperor and explictly declares himself the heir of Charlemagne ?
More in detail, Napoleonic France still annexes French-speaking areas of Walloonia, Savoy, and western Switzerland. Germany becomes a vassal state including Austria, Silesia, Prussia, and Bohemia-Moravia, Italy becomes another vassal state including Naples, Trento, Istria, and Dalmatia. Poland is recreated in the territories of 2nd-3rd Prussian partition and 1st-3rd Austrian partition as a vassal kingdom, Hungary becomes an independent kingdom in its traditional territories, including Croatia. Spain remains an independent vassal under its own dynasty, and is given Portugal.
If he takes steps to let each non-French state have a reasonable amount of internal autonomy, including administrators from loyal local elites and language rights, and implements a economic policy that fosters the industrialization of all the areas of the Empire, can this become a vehicle for stable unification of western-central Europe, and long-term success of the Napoleonic Empire (of course, sooner or later, the Empire would have to return to a liberal political system, if it wants to endure, but there are signs that a successful Napoleon could be willing to do this, in his last years), the British and the Russians successfully kept at bay ?
What other reforms and policy changes would Napoleon have to implement, in order to ensure the success of his neo-Carolingian empire ?
He would still need an heir, of course, but he may just as well do as IOTL and claim a spouse from the deposed Habsburg family, it would add legitimacy to his claim of the HRE title.