What about Ranulf II of Aquitaine living longer? Say, the POD is that he evades the attempt of Count Odo of Paris to poison him and manages to reign another fifteen or so years as King of Aquitaine, long enough to stably pass the Crown to his legitimate son? Correct me if I'm wrong, but didn't his death leave the Aquitainians with no choice but to accept Odo as king, ending the independence of Aquitaine?
The problem is that Eudes, at the contrary of some of his predecessors, have the real control of both the core of the kingdom and of the royal treasure. Not talking about his supportive base.
Furthermore, he really wanted to take Aquitaine, for strengthening his power but also to crush the carolingian remnants they were there and that could dispute his legitimacy.
More important, Eudes had the military power. At these times, when you were king, you had to renounce every other dignity you have : count, general, etc. But Eudes kept this power by giving it to his brother Robert.
So the first factor to remember : Eudes was particularly powerful.
Second, before the death of Ramnol he giving him many domains. Ramnol couldn't take himself without angering the aquitain nobles, but Eudes CAN do that as a king. So Ramnol accepted and renounced to his title, in exchange of the...quite new and curious one : "Duke of the bigger part of Aquitaine".
I'm not saying that Ramnol was particularly happy of this agreement, but Eudes excited his allies within Aquitaine to overthrow the duke and even if Ramnol escaped poison, i doubt he could have escaped and defeated the Adalelmids. Not without renouncing to any royal pretension.
If you want to use the Ramnulfid for an Aquitain kingdom, you'll probably have to make Eudes killed during the siege of Paris, and to ensure that the carolingians were too busy with the wars against Germany and their own dissenssion to busy about Ramnol.
For Ramnol, you have to make him LESS ambitious. If he thing his current desmenes are enough, and if he renounce to them (maybe using a brother or an ally to rule them indirectly) he could put himself in the tradition of carolingian kings.
You could have Guilhem Aigret of Aquitaine or his son married to Dolça II of Provence who both died as a child, it might unite Aquitaine and Provence for a later POD.
Another is for the Capetians to rule Aquitaine instead of Northern France.
I don't get the point : a change of dynasty isn't going to change anything to the political, economical and feudal situation of Aquitaine and France.
And for the union...It's not working that way, not like in a CK game : the nobles that formed the base of feudal power wanted to keep their leader, and not being part of a more important duchy/kingdom etc. because it could mean that the new liege would be too powerful for them, making their autonomy weakening.
Even in the Late Middle Ages, you have kings and great nobles just giving up too further lands they inherited or owned because they couldn't hope rule them against the will of local nobles.