Tobit said:
I like doing this in Crusader Kings. The problem is France tends to come off of me even though I inherited the duchy fair and square.
No wonder. Aquitaine was, by far, the largest and richest Medieval fief of France : it basically included everything South of the Loire river except for the County of Toulouse, the other major fief of southern France.
Ever since the days of Clovis, the Franks (and later the French) tried everything they could not to lose Aquitaine. It could have even become a part of the French kingdom earlier than 1453 had everything turned okay during Louis VII and Eleanor's marriage, or if Eleanor had married the eldest son of Louis VI (a man called Philip who accidentally killed himself in 1131).
A merger of Aquitaine and Navarre might be possible via a dynastic marriage... But I think the Franks/French are going to try everything they can to keep Aquitaine as part of the French Kingdom.