I really think that the majority of royal jewels is expected to go to Princess Mathilde; because she will need it. Princess Astrid and her family will be again on a more shadowed position once Philippe is King. And once Astrid has died, her children will not use tiaras and major jewellery. IMO of course.
About the questions going on here on the forum about the minor jewellery collection. If you compare to the UK, a lot of Queen Elizabeth's tiaras have a very important royal provenance and the fact that Queen Mary bought a lot of jewellery from befriended monarchies that are now gone. Belgium is a very young monarchy with some sad generations: the daughters of Leopold II got their mother's jewellery on her death, but they married into other families/monarchies and so the jewellery went over to them. Queen Elisabeth of Belgium had jewellery which she gave to her daughter (to Italy) and to the Prince Regent (who sold it). The jewellery that went to Queen Astrid (few) through Leopold III was actually taken by Princess Liliane or to Joséphine-Charlotte (to Luxembourg).
A short story about a recent meeting I had with Princess Astrid; I heard her saying to a member of the public quite some months ago now that she wore a jewel from her beloved grandmother Astrid; and the Princess said "It is the only thing I have from her". I think it was a bracelet she wore during one of the evening in the Palais des Beaux-Arts for a concours Queen Elisabeth (music concours). I do not mean that I have a proof here that all is gone, not at all, I am not an insider, but I really think that in the present line of the monarchy, no major jewellery is included from past generations.
What we see now with Mathilde is that she adds a nice touch of modern jewellery to her collection, which I really like. Her sister Elisabeth produces loads of jewellery for her and I personally think that Natan has a jewellery line of costume jewellery and demi-precious stones which she from time to time chooses to add to her outfit. I walked past the Natan store some time ago and noticed that there is a lot of "Mathilde-like" earrings and bracelets in the shop; not that she wore it, but a bit in the sae 'colourful' style that Mathilde choose some years ago.
In the future, I really hope that Queen Fabiola will leave her jewels to Mathilde. And I think that she has the style to combine the antique older things with perhaps a flashy and modern dress. I think we will be pleased in the future with Mathilde.