If Maximiliana of Bavaria doesn't die in childhood, she could be a candidate. And since she didn't marry historically there's not as many factors to consider.
Maximiliana was engaged to her cousin, the duke in Bavaria. Her dying caused him to marry Ludovika instead, and neither partner was happy. She because it was the poorest of matches of her sisters (who wed kings or heir presumptives) and him because he preferred his mistress' company and because the match was forced on him.
A Beauharnais match (esp since this boy would be inheriting the kingdom of Italy as per the agreement by which Napoléon was crowned king of Italy, namely that it would go to a second son and failing any extra issue, to Eugène de Beauharnais) or one with an Italian family (Maria Cristina of Savoy, one of her d'Este nieces, etc.) might make sense.
Failing this, I imagine a cousin or someone with equally Napoléonic origins (Clary, Bernadotte, Beauharnais, or one of the marechaux's families)