First of all, they look nothing alike apart from being the same species. Here's Ashla, and here's Ahsoka. The markings on their faces are very different, and I don't think that's just a trait of the animation.
Second, ROTS and the end of the war (Utapau) are three years after AOTC and the start of the war (Geonosis). The timeline of the war is somewhat hazy due to the overlapping of the shows, novels, comics, and video games (to me, anyway), but this new movie/show takes place after Anakin is knighted (look at the old show). This new movie/show more or less takes place when he's running around saving the other Jedi, before he and Obi-Wan reappear with their new haircuts, hence Anakin's long-but-not-that-long haircut seen in the new movie/show, so it's at least a few months after AOTC. It's my understanding that the members of the Bear Clan (the younglings in AOTC) were between four and eight in AOTC, and Ahsoka is 14 when she becomes Anakin's Padawan, meaning it's impossible for Ashla and Ahsoka to be the same character.
Though as you said, it's an interesting idea, but ultimately an incorrect one.