Let's say achieve the survival of the French Empire as it was at its maximum extent in OTL, or something close to it (maybe a Republican version?). And of course, lasting peace with both Britain and Russia.
The 1812 borders are too wankish already (especially in a republic, they would be far too multiethnic to work! I am not sure if French speakers would even be the majority there). The trap Napoleon laid for himself was the continental blockade. First of all, it didn't work, then it made him hugely unpopular in many regions and bore little benefit in comparison. Then it undermined the otherwise quite well working reign of Louis in Holland and led to the extreme annexations like Northern Germany or Illyria. And most importantly, it ruined the otherwise quite good relationship to the Tsar and helped lead to the war, which might otherwise have been delayed or prevented.
As long as there is peace with Russia, Austria (aligned by marriage and very cautiously led by Metternich) would be quiet. Prussia has been far too weakened to be a factor without Russian involvement.
Peace with Britain is IMHO the hardest part -
but if Napoleon manages to find a solution in Spain, it will be harder for them to disturb L'Empire.
Maybe the key of the problem is Napoleon himself. I am not sure if his ego allows him to not provoke war every other year.