Well, it should be linked to the kingdom dissolving upon death and the unifing franks decision. Soissons is already titular, and Lugdunensis is a good name for the roman kingdom. Soissons should not be able to be controlled by the Franks, and if Syiagrius looses (by getting invaded, rebellions or whatever) the kingdom should be destroyed automatically, maybe reformable be a gallo-roman (EDIT: Not needed as Lugdunensis already models that).
I would also love to take a look at the mod Britannia's dynamic dejure, tribal and titular titles formable by decisions.