I think far more nations should be able to form lotharingia.
At the moment it is just Burgundy and Lorraine as far as I know, which is odd as Lorraine is merely the Successor state to Upper Lotharingia, but there is no representation of Heirs of Lower Lotharingia.
Brabant for example most likely has the strongest claim to reforming Lotharingia if it were to gain independance from Burgundy, with Burgundy probably also using this exact claim to the name as Dukes of Brabant.
en.wikipedia.org
Further basically all these states have some form of claim over it:
Cologne, Liege, Utrecht, Gelre, Cleves, Hainaut, Holland and Berg (and Julich), are all tags that should have some form of way to maybe reestablish it.
Holland and Utrecht maybe being the exception to this as they should probably be incentivized to become Netherlands.