I'm not sure where Lucius would ally himself but I don't see him making the bid for power personally. I loved what Marcus said about the Legion, that most of them follow the man and not the vision. If the line of succession is set on Lanius, good ol' Vulpes would know how much that screws him (immediately) and the rest of the Legion (eventually) and the Courier would see the unique opportunity of the situation. Maybe goody-good Courier looking to reform the Legion, and evil-fuck Courier looking to consolidate power over the Mojave or NCR allied Courier who could swing either way but wants to use the moment to weasel in and hasten the Legion's demise...

So many ways this could go! I do hope someone adopts this propmt or I may just have to end up self-filling!