My vote is someone from the "powerbrokers" category, like Schmidberger or du Chalard: These men have beeen running the SSPX behind the scenes (at least with regard to Roman relations) for decades. Schmidberger was a longtime friend of BXVI (so much so that +Lefebvre had begun working to ensure he was not reelected in 1994). And du Chalard has been the primary liaison between Rome and Menzingen for over three decades. Expelled as superior by the Sisters of the Cenacle (i.e., SiSiNoNo), he is Rome's kind of liberal. Both are GREC supporters, overt ralliests, and have been selling the deal for decades. Rome will feel most comfortable with this kind of candidate. Lorans (a GREC co-founder) would be my dark horse, and I think Celier's reputation is too flagrantly suspect to be viable.
If the candidate comes from outside this category, my money is on LeRoux (who is already being replaced at the seminary by Fr. Goldaddy, in preparation for a new role in the SSPX).