Taking into account Alizée's pregnancy and the fact that she wanted to 'take a break' after "En Concert", I believe (unless Mylène would have somehow managed to convince her to return to the Studio immediately), that the third collaborative Album wouldn't have been released until 2007 at the earliest.
Here's why:
1. Mylène began recording "Avant que L'ombre..." in early 2004, originally planned as a double Album. I'm fairly certain that this was in the works for quite a while.
2. After, Laurent Boutonnat began work on "Jacquou le Croquant" (a film, for those who may be unaware), which released in January 2007.
Factoring all of this in, work on Album number three wouldn't have commenced any earlier than the actual work on "Psychédélices" did, that we would eventually get.
It is of course impossible to say what the lyrical content would have been or what form Alizée's image would have taken, although I will say that Mylène is fairly big on continuity so I'm pretty sure they wouldn't have strayed too far from the established and simply found a way to 'humbly' work themes such as marriage and motherhood into it and make it look progressively, if nothing else.
So, in any case, the best point of reference for how that third Album might have sounded like would be Mylène's 2008 output:
"Appelle Mon Numéro" (Translation: "Call My Number") serves as a good example, as, even though the song is very much Mylène's, it is one of those titles that I believe Alizée could have convincingly sung as well:
Another example may be the following:
"Drôle de Creepie" was a song written by Mylène with music by LB for her niece, Lisa. A sort of "Prototype Alizée". It was a one-off and yet fairly successful single, also released in 2008.