What about something along the lines of the Rhineland?
Since the actual border areas were only recently (past 300 or so years) finalized, I'd say that any pre-EU realm still would not carry such a name historically. Now, if I were playing and I was playing a non-rp type of game, I think something like this would work quite well; especially since the player would more than likely never lose what was gained.
Well Francia Media consisted of Lotharingia, Burgundy/Arelat and Italy; the northern bit once the core area of Austrasia was only known as Lotharingia. Lotharingia wasn't named after emperor Lothar, but after his 2nd son king* Lothar II. (*= King of Lotharingia, though every Carolingian king was a king of the Franks).
While true, @mudcrabmerchant was trying to come up with some non-Lothar based names in post #16, so that was my goal in fielding alternatives to Lotharingia. I prefer Lotharingia for the 867 start in place of Middle Francia; for the actual 769 start I'd personally divide the area into Austrasia, Arlet and Burgundian kingdoms. Off the top of my head, I can not remember if a Catholic pope sanctioned Lotharingia or if the only legitimacy it had were the various treaties. I know a Pope did sanction the Austrasia, Arlet and Burgundian kingdoms so that is why I prefer them to be both de jure and de facto kingdoms in the 769 start.