If he captures them, they are worth something at the peace table. Given what was offered in 1813, and Nappy not taking it, I have my doubts peace will happen even if he gets a slightly better deal.
Alexander might be more willing to sue for peace, since Russian thrones tend to be pretty insecure, and no Russia in the war changes things.
Also, much depends on whether this butterflies Poniatowski's death- I think Napoleon respected him highly, and he's alive I don't think he sells out Poland as easily.
Józef Poniatowski died at Leipzig, so with a POD before the battle there's a chance he could survive. Personally I like the idea of Józef marrying Princess Maria Augusta of Saxony, the only child of Frederick Augustus of Saxony & Warsaw and the so-called Infanta of Poland, and jointly ruling over a smaller Poland.
There was not only Alexander I of Russia but also Frederick-William III of Prussia and their heads of staff who rescapés being captured by an inch.
So here's my assessment of the situation.
Napoleon had already been strategically rolled back from the Niemen to the Elbe. He could not regain Poland and the Niemen.
But tactically, although lacking cavalry, he more often bested his opponents in the 1813 campaign.
So if he captures the political and military heads of Russia and Prussia, the coalition is beheaded. It will be the miracle of the House of Bonaparte the same way there was a miracle of the house of Brandeburg in 1762.
My guess is that either the coalition retreats and does not fight at Leipzig or that it is severely defeated.
There is going to be a peace treaty rather favourable to France.
France will have to give-up its hegemony over the Rhine confederacy because that's a condition that Russia, Austria and Prussia want. Either the confederacy becomes independant or it becomes "protected" and guaranteed by the great powers.
Bavaria will hand Tyrol back to Austria and Austria will regain Illyria.
Russia and Prussia will break Poland up,
France will hand Spain back to the spanish Bourbon.
France will restore a Dutch independant kingdom north of the Rhine.
France will agree that after Nappy's death, the crown of the north italian kingdom will be held by someone different from the emperor of France.
France will get some compensations, maybe western Switzerland and Milan, plus some colonies : dutch Indonesia and some spanish colonies (maybe Argentina if if France can succeed in the conquest Britain failed).
Matteo, I think your WAY overestimating the coalition's bargaining power. If the King and Tsar, along with their command staffs, are captured and the French win a decisive victory at Leipzig, then the Coalition will probably collapse.
If we're assuming the Coalition is routed and whats left of the armies retreat, either to Bohemia or breaks up along national lines, then Napoleon's in the position to regain the tactical advantage. Breaking down your ideas for the peace:
1. None of the three eastern powers would be in a position to dictate anything about the Confederation of the Rhine. Napoleon still controls most of Germany and I'd assume Bavaria, Saxony and the other states are still loyal. So on route there unless the Coalition recovers, beats Napoleon elsewhere and moves into Germany.
2. Tyrol and IIllyria MIGHT be returned to Austria, if the Austrians can quickly make peace, switch sides or if their army is relatively intact. If their army is crushed then Austria's out, as this was basically their last role of the dice: they were running out of manpower.
3. Poland's going to be the main sticking point. Remember, Prussia's King and Russia's Tsar are in Napoleon's custody: would either power be in a position to fight on here? OK maybe Prussia but Russia, the absolute monarchy deeply dependent on the Tsar to keep the government running? I doubt it.
3 and 4. WHY would Napoleon, who's just pulled a coup de grace, give up the Netherlands and Spain? The coalition, on the other side of Europe, would be in no position to dictate such terms, even with an ATL victory at Leipzig. Nappy would laugh in their faces.
5. Legally the Napoleonic Kingdom of Italy was still set to go to Eugène de Beauharnais, Napoleon's former step-son. While Napoleon had hinted at wanting to leave both France and Italy to the King of Rome, nothing had legally changed by 1813.
6. Finally, only Britain's in a position to give over colonies to France, which I can't see them doing at this point. Although if a permanent peace is established in Europe between Napoleon and the sixth coalition forces, then we could see London finally come to the barging table.
Britain would probably get an evacuation of Portugal (after all, Napoleon had been driven out of Portugal in 1811, so its realistic for Nappy to give on that), an end of the Continental system, recognition of British control over Malta, the independence of Sicily and Sardinia and maybe some kind of compensation over Hanover and to the House of Orange. On the flip side, Napoleon would get Britain to withdraw its army and support from Spain, no more financial support for French enemies and maybe some of the occupied French colonies returned.
While some of my ideas for a permanent Napoleonic peace might be wrong, I think they're more in touch with the situation on the ground and what Napoleon would actually accept.