As you said, it's still very unlikely, but more likely than the Sardinian match. Louis-Philippe probably was a guy who was willing to do anything for power. So it is feasible to see the Orleans match with Napoleon II. Now whether or not it would be beneficial in the long run however, I do not know.
Frankly, it's a toss up between the following:
- Louise d'Orleans (OTL 1812-1850): daughter of Louis Philippe III, Duke of Orleans. OTL Married Leopold of Saxe-Coburg-Gotha/Leopold I of Belgium
- Mathilde Karoline von Wittelsbach (OTL 1813-1862): daughter of Ludwig I, King of Bavaria. OTL Married Ludwig III, Grand Duke of Hesse-Darmstadt
- Theodelinde von Wittelsbach (OTL 1816-1817): daughter of Ludwig I, King of Bavaria. OTL died in infancy
- Maria Theresa von Habsburg-Teschen (OTL 1816-1867): daughter of Charles, Archduke of Austria and Duke of Teschen. OTL Married Ferdinand II of the Two Sicilies.
- Hermine von Habsburg-Lorraine (OTL 1817-1842): daughter of Joseph, Archduke of Austria and Palatine of Hungary. Unmarried OTL.
I'd say Hermine would be to low ranking but Archduke Karl's daughter could be interesting. Napoleon did have respect for him as a general so an alliance of their children could fit in his thinking. But of the Bavarians I'd say Mathilde is more likely.
I can see a Russian bride being one of the conditions of a peace agreement. That is the exact way he married Marie-Louise, and one of Napoléon's long-term goals would be a stable Franco-Russian alliance as a basis of stability in Europe. Marrying yet another Hapsburg would be a bit consanguine (something even the Hapsburgs had stopped to do for about one century ;-) ) and Prussia is essentially destroyed as a state, so the Hohenzollerns are out. Spanish and Sicilian ideas are fun but maybe too Bourbon (but that would make for a fun royal competition in France if the legitimist line of Artois eventually dies out, between the Spanish Bourbons, the Orléans, and the Bourbonapartes). Bavaria is an interesting possibility but maybe too minor, and already dependent enough on Napoléon.
What about an ATL Portuguese Infanta? While Joao and his wife did detest each other, a short reconciliation could be possible and their last child was born in 1806, so not impossible to consider. I can see Britain demanding the restoration of Portugal as a term in peace negotiations and Napoleon might be willing to do so , if only to get Britain out of Spain. A marriage concession could be a possible demand of the Emperor.
Or, if the POD was no deposition of the Borbóns of Spain, then a Spanish bride could be a good choice. Have Fernando VII remarry and produce a daughter. She'd be of an age to marry the King of Rome.
These four will have my support if you can prove that butterflies wont affect them.
What about Grand Duchess Elizabeth Alexandrovna of Russia? She is the younger child of Alexander I of Russia and his wife, Elizabeth Alexeievna (Louise of Baden) In OTL she died in 1808, but if Napoleon won and had a French doctor care for his Russian hostage, spots the signs of an infection and treats the young duchess; in OTL the infection was just blamed on teething.
Or Princess Vilhelmine Marie of Denmark? The youngest daughter of Frederick VI of Denmark, who made Denmark the most loyal ally of Napoleon, it would seem a fitting gesture to make a loyal ally's daughter Empress.
Can't see Alexander giving his daughter as a hostage. But a Surviving Elizabeth or an ATL sister (after all her mother was still young enough to have another child, one that could survive to adulthood) would make an interesting match.
As for Vilhelmine, I'd considered her as well but she did die childless and was a Protestant. Napoleon wasn't that religious but I think he would recognize the importance of a Catholic bride. Vilhelmine would have to convert.
I like an ATL Grand Duchess of Russia or Iberian Infanta the best. Both are ranking enough to be considered "worthy" of the Emperor's son and any of them could cement either an alliance (Spain) or peace (Russia and Portugal). Plus they have the added benefit of having no close relation to Napoleon II, avoiding inbreeding.