My guess:
Kate: the Halo (or the Lotus)
Meghan: the Diamond Bandeau
Anne: the Double Meander (which we only had seen once
) or QM's Fringe
Zara: the Greek Meander
Autumn: the Festoon
Beatrice: her future wedding tiara (btw I don't think it will be the York and hope it will be QM's Fringe)
Eugenie: the Greville Kokoshnik
Sophie: her wedding tiara
Louise: the Five Aquamarines
Different stories if they are given new tiara (which I don't think it will happen).
It would be nice to see some grander tiara, but I also don't think it is necessary for the ladies to get a major upgrade for the coronation. After all the coronation is about the new monarch. I think for the rest ladies they will just wear what they had already accessed. Grander tiaras may be borrowed and worn for the gala or reception?
Other reason is that they also have to wear the coronet. Grander pieces may be too much and heavy (I can't imagine wearing the Lovers Knot with coronet, just too too too busy). And I don't think it's a good idea to wear some new-to-you tiaras which you don't have enough time to explore to such important event.
Yes the Halo is my first choice for Kate. It's relatively small (and I insist that it can be substantial if the wearer gets the right hair, see Margaret), but it goes well with the coronet (again, see Margaret, I think their coronets will be same type?). I like the idea that wearing tiara with "coronation history" to coronation. The Lotus is nice too but I would prefer the Halo as its shape and style suits Kate better. And maybe she will get some other major jewels, e.g. the Godman necklace or the Greville earrings? Somehow I think that if she really wear a new grand tiara, it would be some long-forgotten tiara in the vault and it becomes her another signature piece as PoW.
I am hoping QM's Fringe for Anne (or Beatrice as her wedding tiara) as I hope its next wearer will be, also like its previous wearer, princess by birth. (OT I would love to see Charlotte wear it in far future too.)
For Sophie, I know her wedding tiara is not visually pleasing but it has the most historical value......I don't think she will get a more significant new loan as she already had three, so let's hope she will alter or remake it. And please also alter the Five Aquamarines and take away these wires on the base, it can be a nice tiara for Louise.
An OT question: will the Cambridges automatically become the Prince/ss of Wales once Charles become King or they have to wait until Charles create them PoW?