I also am on board with more Yvette but have been thinking about how the tier 3 girls can get a boost without too much labor from the Developer.
Obviously, this would be for the future, and maybe far in the future as the Creator is busy trying to tell the story and secrets of the island. Any writer will tell you that it isn't a good idea to interrupt this process. Plus, what you are asking is like going backwards since it is filling in parts that the other girls already have. This can be a boring, repetitive process.
Still, it might be possible to add 1 girl to each update for 3 or 4 updates.
I have suggested Delizia, not because she's my favorite (although she wins every Best Butt contest that I judge), but because a lot of the design work has already been done. We've seen her do yoga poses. Getting a look at her boobs during the beach walk would be fairly easy to add. The pool distractions don't require many renders. Most of all, there would be very little writing for the Designer to do.
Compare this with Natasha: since Natasha already has a lot of the content the other girls have, extra content for her requires a lot more work. Just think how much writing goes into the Hot Shots game!