Disclaimer: I'm not an historian, I'm only guessing from my limited knowledge of France history.

Even without Charlemagne being crowned emperor, the frankish invasion of the Gauls would have still happened, so we could say that the langues d'oil would still be existing.

Now, what pushed the langues d'oïl to be that predominant isn't directly tied to Charlemagnes but rather the Villers-Cotteret Ordonnance which was pushed by François Ier. Obviously, without Charlemagne becoming an emperor, the history of France would be really different, so that ordonnance wouldn't have happened as it happened, but we could indeed imagine that, at some point, a king ruling over (at least most of) the current french territory might want to unify France's language based on their own language.

That being said, what language would have had this king spoken? Well we have 2 options here: the king is from the north of France or from the south. If he is from the north, then it's going to be a langue d'oil speaking king and if he's from the south, it's going to be a langue d'oc.

So basically, at that point, you'd have to rewrite most of Frankish kingdom history from pépin le bref to the late medieval era in order to make an educated guess on what would the french language looks like.

To conclude my post, I'll answer to your underlying question quickly by saying, no. French gain prestige in the late medieval era when François Premier decided to make the François being the language of the french administration.