But all of this depends on the Coalition breaking and them suddenly being OK with the man who they called an outlaw to the worlds and had treated almost all of them like vassals (breaking up their countries, taking away land to give to his siblings, invading them or blockading them) being in charge of France again (and giving him time to rebuild France's war machine - which he WOULD use again, Napoleon's ambition to be the dominant force in Europe never left him). I can't see that happening. Eventually if given time, he would try to expand France's frontiers - particularly in the areas he had previously annexed.
Also Austria will never return his son and wife even if the Coalition breaks and Bonaparte was no longer in a condition to invade Austria to get them back. So was supposed to be the heir to the reborn Empire - the weak Joseph or the Austrian-raised German-speaking King of Rome?
All this of course is moot unless the Coalition actually does fall apart. If Napoleon does somehow manage a victory in 1815 In terms of an heir Napoleon could I suppose marry Marie Walewska and make his son with her, Alexandre Joseph the heir. Eugene de Beauharnais might be a possibility though he had been excluded from the succession. Possibly a candidate for regent however. Maybe one of Napoleon's brothers but they were all rather ineffectual.
In any event a peace after 1815 with Napoleon remaining as Emperor of France would be unlikely to last, And when Napoleon dies there is the succession problem to resolve.
In all probability that Napoleon will be defeated and overthrown by the end of 1815 so these issues will never arise as the Borbons will return.