Louis-Philippe's son, the Duke of Nemours was popularly considered before Leopold, but the Great Powers weren't sure because he was French. Louis-Philippe also rejected the offer on his son's behalf.
Since the problems of chosing Nemours ITTL are still the same, I think that a good candidate would be Leopold's eldest brother, Prince Ferdinand Georg of Saxe-Coburg and Gotha. He was already a Catholic at the time of the Belgian Revolution (having converted 15 years earlier in order to marry a rich Hungarian heiress) and had important political connections with the Habsburgs too.
Interesting enough, IOTL his eldest son - also named Ferdinand - married Maria II of Portugal, becoming king-consort of that country. ITTL he would be the heir of the Belgian crown, and so wouldn't be married to her. We probably would see his younger brother August marrying her instead, or a prince from other noble house would be chosen as king-consort of Portugal and install his dynasty there.