Leopold as emperor/king of Mexico would probably marry someone Spanish or with a Spanish mother. Assuming Fernando VII still forbids his brothers from accepting the crown, that limits it to a Portuguese, Parmese or Neapolitan infanta (an Austrian archduchess would be deemed "too good" for a mere prince of Coburg and really the only way I could see it is if D. Leopoldina survives, and Austria has an interest in Mexico for some reason). Its important to remember that by the time of his marriage to Louise d'Orléans the persons who had helped arrange his marriage to Princess Charlotte (Ekaterina Pavlovna, Karoline of Brunswick, and the duke of York) were all dead. Leopold marrying Louise was something of best-for-both: if her dad was deposed by a new revolt or Charles X came back, France still had a tie to Belgium (and England) as Louise was the only one of the Orléans clan that Madame Royal and the duchesse de Berri both still held in any esteem after July 1830. Leopold's throne was unsteady until the end of the decade, so if he was deposed by the Dutch he had a place to land (Paris, since the British despised him. Mostly they saw him as a parasite because he was still drawig the allowance he'd been allotted as Charlotte's husband, still kept house at Claremont etc, all of which the average Briton was paying for. ISTR from one bio that many Britons were thoroughly disgusted at him using moneys from this allowance to pay the expenses of his court in Brussels. This opinion of a parasite was the legacy Albert had to deal with and why he was "bound and gagged" far worse than Leopold had been). To my mind all the Cobirg men (Leopolds I and II, Albert, Fernando II of Portugal, Ernst I/II of Coburg, Fernando's brother and nephew) were bloody parasites that should've been dragged out into the street and shot. Leopold I would've done no differently with Charlotte what his nephews did with Victoria/Maria II/what they would've done with Isabel II had that marriage gons through: keep her pregnant so that they can rule. And if she dies as a result (Maria II), hey, so much the better.